[WIP][multibyte] Add multibyte array store instructions.

[brendandahl]

Prototype implementation of the new multibyte array proposal that reuses the existing memory instructions. The syntax for the new instructions is as follows: <i32|i64|f32|f64>.store type=array

Some spec related TODOs:

• Decide on new instructions vs existing memory instructions with memarg flag.
• If using existing instructions:
  • What does the wat syntax look like? e.g. i32.store type=array x y or i32.store array x y or ???
  • Do we support immediate offsets? - Do we support i32.store8? (effectively the same as array.set on i8)
• Do we support atomic instructions?

Some implementation TODOs:

• Add feature flag.
• Add v128.store.
• Go through the various visitArrayStore functions and implement them or make sure the copied versions makes sense.
• Add fuzzing support.
• Make it so there's only one WasmBinaryReader::getMemarg
• Add JS API