GC: Fuzzing support for i31, assuming i31.new is constant

[dcodeIO]

Integrates i31ref types and instructions into the fuzzer, by assuming that (i31.new (i32.const N)) is constant [1] [2]. Might make sense to delay landing this until we know more.

[dcodeIO]

Invocations so far:
   FuzzExec: 12024
   CompareVMs: 3277
   CheckDeterminism: 1030
   Wasm2JS: 2870
   Asyncify: 3052

ITERATION: 14108

Going to do more fuzzing tomorrow.

Another run that unfortunately hit #3149 early on and I didn't notice:

Invocations so far:
   FuzzExec: 6658
   CompareVMs: 1771
   CheckDeterminism: 573
   Wasm2JS: 1533
   Asyncify: 1643

ITERATION: 7808

Another:

Invocations so far:
   FuzzExec: 15288
   CompareVMs: 4240
   CheckDeterminism: 1318
   Wasm2JS: 3549
   Asyncify: 3858

ITERATION: 17977

[tlively]

Thanks! LGTM

[dcodeIO]

Invocations so far:
   FuzzExec: 20829
   CompareVMs: 5655
   CheckDeterminism: 1773
   Wasm2JS: 4884
   Asyncify: 5214

ITERATION: 24375