[C API] Some BinaryenString* functions declared in binaryen-c.h without implementation

[leobuskin]

• BinaryenStringIterNext
• BinaryenStringIterMove
• BinaryenStringSliceIter
• BinaryenStringNewSetTry
• BinaryenStringWTF8Advance

My FFI tooling breaks on those. Looks like there's no implementation, just declaration. Am I missing some compilation flags?

I've tried to find related commits (in case they were removed), but GitHub search wasn't helpful

[kripken]

Those do appear to be missing implementations. Declarations were added in 0821cd1 but I guess those were forgotten. It would be good to add them now if someone has time.