fuzz: fully fuzz Wasmtime against the spec interpreter

[abrown]

https://github.com/bytecodealliance/wasmtime/pull/3124 adds the initial steps for fuzzing Wasmtime against the official WebAssembly spec interpreter. There are several pieces to finish for more complete coverage:

• [ ] After execution, compare the memory contents of Wasmtime against the memory contents of the spec interpreter
• [ ] After execution, compare the global values of the module in Wasmtime against those in the spec interpreter
• [x] Add support in the wasm-spec-interpreter crate for v128 types; this implies turning on SIMD for the differential_spec fuzz target as well as merging the Wasm SIMD proposal into https://github.com/bytecodealliance/wasm-spec-mirror.
• [ ] Add support in the wasm-spec-interpreter crate for ref types

[abrown]

SIMD support added by @conrad-watt in #3843.

[alexcrichton]

This is more-or-less done now modulo reference types, but that seems ok for now so I'm going to close.