Typechecker (MGU.hs): fix unergonomic checking of function applications

When checking a function call, and it's wrong, we expect the error to
be reported as the arguments having the wrong type, not the function
being the wrong type for the arguments we provided.

Therefore, check an application by matching the argument against the
function, not the function against the arguments. In particular, we
should generate a fresh argument type and unify it with the type of
the function, _then_ unify it with the argument expression, not the
other way around.

Someone stepped on this today internally and it was confusing all
around.

Add a test case in test_type_errors.

intTests/test_type_errors/err002.log.good

@@ -0,0 +1,12 @@
+ Loading file "err002.saw"
+ err002.saw:5:44-5:57: Type mismatch.
+    Mismatch of type constructors. Expected: String but got ([])
+    internal:1:1-1:7: The type String arises from this type annotation
+    err002.saw:5:44-5:57: The type [LLVMType] arises from the type of this term
+
+    Expected: String
+    Found:    [LLVMType]
+
+    within "spec" (err002.saw:4:5-4:9)
+
+FAILED

intTests/test_type_errors/err002.saw

@@ -0,0 +1,8 @@
+// llvm_alias takes a string, pass it a list of types instead
+// (user meant to use llvm_struct_type and used llvm_struct,
+// which is a deprecated name for llvm_alias)
+let spec = do {
+   input <- llvm_fresh_var "x" (llvm_alias [llvm_int 32]);
+   llvm_execute_func [llvm_term input];
+};
+

src/SAWScript/MGU.hs

@@ -849,12 +849,13 @@ inferExpr (ln, expr) = case expr of
        (body', t) <- withPattern pat' $ inferExpr (ln, body)
        return (Function pos pat' body', tFun (PosInferred InfContext (getPos body)) pt t)
 
-  Application pos f v ->
-    do (v',fv) <- inferExpr (ln,v)
-       t <- getFreshTyVar pos
-       let ft = tFun (PosInferred InfContext $ getPos f) fv t
-       f' <- checkExpr ln f ft
-       return (Application pos f' v', t)
+  Application pos f arg ->
+    do argtype <- getFreshTyVar pos
+       rettype <- getFreshTyVar pos
+       let ftype = tFun (PosInferred InfContext $ getPos f) argtype rettype
+       f' <- checkExpr ln f ftype
+       arg' <- checkExpr ln arg argtype
+       return (Application pos f' arg', rettype)
 
   Let pos dg body ->
     do dg' <- inferDeclGroup dg