[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.

[GulgDev]

Implementations of BinaryenStringIterNext, BinaryenStringIterMove, and BinaryenStringSliceIter were added in 0821cd1, but for some reason they were later removed.

[kripken]

These would be good to add, but note that we now have a StringLifting pass. One can emit imported strings, then lift them into the String* instructions, so it is less important to emit String* instructions directly (in fact, most toolchains might prefer not to, since VMs don't support them).