Binary encode from back-to-front

Our current front-to-back binary encoder has a subtle performance
problem:  In order to allocate the correct number of bytes for the
size of a sub-message, it needs to know the final size of the sub-message.
This results in recursive calls to size each message, leading to overall
performance that is quadratic in the depth of nesting.

This is not usually a serious problem since few people have messages with
more than 2 or 3 layers of nesting.

This PR changes the encoder to encode from the end of
the buffer back towards the front.  This allows us to
write the size after we finish each sub-message, avoiding
the recursive sizing calls.  With this version, we have only
one sizing traversal in the initial top-level encoding request
in order to properly allocate the output buffer.

Working from back to front does mean that individual fields get
written in the opposite order.  This is _mostly_ not a problem:
Protobuf explicitly states that decoders must accept fields in
any order.  The one exception is for repeated fields, which we
handle here by iterating the arrays backwards inside the encoder.

A few cases where order might matter:
* Unrecognized enum cases in repeated fields are treted as "unknown"
  fields which means that re-serializing puts them into a different place.
  Since this code puts the unknown fields at the beginning of the buffer
  rather than the end, this means that we've changed the resulting order
  after deserializing/reserializing.
* The conformance test has one test case that verifies merging behavior
  and seems very sensitive to the order of fields.  I suspect this is
  a bug in the test, but need to check further.

Sources/SwiftProtobuf/BinaryReverseEncoder.swift

@@ -0,0 +1,156 @@
+// Sources/SwiftProtobuf/BinaryReverseEncoder.swift - Binary encoding support
+//
+// Copyright (c) 2014 - 2016 Apple Inc. and the project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See LICENSE.txt for license information:
+// https://github.com/apple/swift-protobuf/blob/main/LICENSE.txt
+//
+// -----------------------------------------------------------------------------
+///
+/// Core support for protobuf binary encoding.  Note that this is built
+/// on the general traversal machinery.
+///
+// -----------------------------------------------------------------------------
+
+import Foundation
+
+/// Encoder for Binary Protocol Buffer format
+internal struct BinaryReverseEncoder {
+    private var pointer: UnsafeMutableRawPointer
+    private var buffer: UnsafeMutableRawBufferPointer
+
+    init(forWritingInto buffer: UnsafeMutableRawBufferPointer) {
+        self.buffer = buffer
+        self.pointer = buffer.baseAddress! + buffer.count
+    }
+
+    private mutating func prepend(_ byte: UInt8) {
+        consume(1)
+        pointer.storeBytes(of: byte, as: UInt8.self)
+    }
+
+    private mutating func prepend<Bytes: SwiftProtobufContiguousBytes>(contentsOf bytes: Bytes) {
+        bytes.withUnsafeBytes { dataPointer in
+            if let baseAddress = dataPointer.baseAddress, dataPointer.count > 0 {
+		consume(dataPointer.count)
+                pointer.copyMemory(from: baseAddress, byteCount: dataPointer.count)
+            }
+        }
+    }
+
+    internal var used: Int {
+        return pointer.distance(to: buffer.baseAddress!) + buffer.count
+    }
+
+    internal var remainder: UnsafeMutableRawBufferPointer {
+        return UnsafeMutableRawBufferPointer(start: buffer.baseAddress!,
+	    count: buffer.count - used)
+    }
+
+    internal mutating func consume(_ bytes: Int) {
+        pointer = pointer.advanced(by: -bytes)
+    }
+
+    @discardableResult
+    private mutating func prepend(contentsOf bufferPointer: UnsafeRawBufferPointer) -> Int {
+        let count = bufferPointer.count
+	consume(count)
+        if let baseAddress = bufferPointer.baseAddress, count > 0 {
+            pointer.copyMemory(from: baseAddress, byteCount: count)
+        }
+        return count
+    }
+
+    mutating func appendUnknown(data: Data) {
+        prepend(contentsOf: data)
+    }
+
+    mutating func startField(fieldNumber: Int, wireFormat: WireFormat) {
+        startField(tag: FieldTag(fieldNumber: fieldNumber, wireFormat: wireFormat))
+    }
+
+    mutating func startField(tag: FieldTag) {
+        putVarInt(value: UInt64(tag.rawValue))
+    }
+
+    mutating func putVarInt(value: UInt64) {
+        if value > 127 {
+	    putVarInt(value: value >> 7)
+            prepend(UInt8(value & 0x7f | 0x80))
+        } else {
+            prepend(UInt8(value))
+	}
+    }
+
+    mutating func putVarInt(value: Int64) {
+        putVarInt(value: UInt64(bitPattern: value))
+    }
+
+    mutating func putVarInt(value: Int) {
+        putVarInt(value: Int64(value))
+    }
+
+    mutating func putZigZagVarInt(value: Int64) {
+        let coded = ZigZag.encoded(value)
+        putVarInt(value: coded)
+    }
+
+    mutating func putBoolValue(value: Bool) {
+        prepend(value ? 1 : 0)
+    }
+
+    mutating func putFixedUInt64(value: UInt64) {
+        var v = value.littleEndian
+        let n = MemoryLayout<UInt64>.size
+	consume(n)
+        pointer.copyMemory(from: &v, byteCount: n)
+    }
+
+    mutating func putFixedUInt32(value: UInt32) {
+        var v = value.littleEndian
+        let n = MemoryLayout<UInt32>.size
+	consume(n)
+        pointer.copyMemory(from: &v, byteCount: n)
+    }
+
+    mutating func putFloatValue(value: Float) {
+        let n = MemoryLayout<Float>.size
+        var v = value.bitPattern.littleEndian
+	consume(n)
+        pointer.copyMemory(from: &v, byteCount: n)
+    }
+
+    mutating func putDoubleValue(value: Double) {
+        let n = MemoryLayout<Double>.size
+        var v = value.bitPattern.littleEndian
+	consume(n)
+        pointer.copyMemory(from: &v, byteCount: n)
+    }
+
+    // Write a string field, including the leading index/tag value.
+    mutating func putStringValue(value: String) {
+        let utf8 = value.utf8
+        // If the String does not support an internal representation in a form
+        // of contiguous storage, body is not called and nil is returned.
+        let isAvailable = utf8.withContiguousStorageIfAvailable { (body: UnsafeBufferPointer<UInt8>) -> Int in
+            let r = prepend(contentsOf: UnsafeRawBufferPointer(body))
+            putVarInt(value: body.count)
+	    return r
+        }
+        if isAvailable == nil {
+            precondition(false)
+            let count = utf8.count
+            putVarInt(value: count)
+            for b in utf8 {
+                pointer.storeBytes(of: b, as: UInt8.self)
+		consume(1)
+            }
+        }
+    }
+
+    mutating func putBytesValue<Bytes: SwiftProtobufContiguousBytes>(value: Bytes) {
+        prepend(contentsOf: value)
+        putVarInt(value: value.count)
+    }
+}