C-style cast
Add function const to MemoryPoolAllocator::Capacity() and Size()

Added unit test for parsing EncodedStream to Document.

Safer implementation of Swap()

Also added documentation, unit tests for swapping Document/Value,
Document/Document.

Miscellaneous cleanups

The error messages in ParseNumber used `is.Tell` to report the
position of the number parsing error.  Depending on the copy
optimization of the current stream, this can lead to different
behaviour (beginning of number vs. position of error).

 * Delegate constant string construction to SetStringRaw
 * Delegate "const Ch*" overloads to GenericValue variants
   of operator[], FindMember and RemoveMember
 * Remove repeated template arguments in nested struct Array

(cherry-picked from ca9b0332d)

Instead of hard-coding the value 0 for the parseFlags in the
various parsing overloads, explicitly use kParseDefaultFlags
to provide more self-documenting code.

@pah

Added GenericValue::Swap()