Implement wasm extensions

[tsavola]

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

• Nontrapping conversions
• Bulk memory operations
  • memory.copy
  • memory.fill
  • data.drop
  • table.init
  • table.copy
• 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).