[C API] Add getters and setters for various GC/Strings expressions (#5037)

Covers CallRef, RefTest, RefCast, BrOn, StructNew, StructGet, StructSet, ArrayNew, ArrayInit, ArrayGet, ArraySet, ArrayLen, ArrayCopy, StringNew, StringConst, StringMeasure, StringEncode, StringConcat, StringEq, StringAs, StringWTF8Advance, StringWTF16Get, StringIterNext, StringIterMove, StringSliceWTF, StringSliceIter.

Author: dcodeIO