Rename index arg

[DuncanUszkay1]

Before we had first class functions, we used this indexArg local in compileCallIndirect. From my understanding, this name no longer makes sense because it is not in fact an index into the function table but actually a pointer into a block of memory which contains that index.

I picked the name functionInstance but I'm not terribly attached to it, I just think that indexArg is the wrong name (Unless I'm misreading the code of course)

This was tripping me up when I was reviewing the code so I figured I'd submit a patch 🤷

• I've read the contributing guidelines

[DuncanUszkay1]

cc @dcodeIO

[dcodeIO]

Makes sense to rename it in context of compileCallExpression etc., yeah, except in context of the call_indirect builtin, where it's still an indexArg. Perhaps, a better name for indexArg in call_indirect might be tableIndex.

[DuncanUszkay1]

@dcodeIO does this change still make sense?