Implement wasm extensions

[tsavola]

Add support for applicable extensions which are or will be used by compilers by default.

• [x] Nontrapping conversions
• [ ] Bulk memory operations
  • [x] memory.copy
  • [x] memory.fill
  • [ ] data.drop
  • [ ] table.init
  • [ ] table.copy
• [x] Sign extension operators
• [ ] Multiple return values

Perhaps need minimal support if compilers or WASI requires them:

• Threads (Gate doesn't intend to support more than one per process/agent).
• Reference types (Gate ABI serializes everything).