2.3: Set and cast functions, performance improvements

• Added set functions: except, intersect, union.
• Added cast function.
• Improved performance of some functions by inlining code: range, rangeTo, max, min, sum.
• Changed iterator wrapping from closure to directly wrapping iterator.
• Fixed integration with Coveralls.
• Fixed builds for PHP 7 and HHVM on Travis CI.
• Code cleanup: short array syntax everywhere.