Enable the memory64 proposal by default

[alexcrichton]

The memory64 proposal for WebAssembly moved to phase 4 early last November which was the final remaining blocker for enabling it in Wasmtime. I've gone ahead and enabled it here with all the other checkboxes being ticked such as:

• Tests - all spec tests are enabled and we have a few tests here and there for memory64 behavior throughout the unit test suite.

• Finished - this proposal's memory-facing bits have been done for quite some time and the final 64-bit table bits have been done for a bit now as well.

• Fuzzed - this is enabled in wasm-smith and additionally has custom fuzzing via the memory_accesses fuzzer.

• API - all APIs related to memory work with u64 values to accommodate 64-bit memories and the 64-bit-ness is reflected in the type of memories.

• C API - the C API's functions for working with memories all reflect 64-bit indices like the Rust API.

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[dicej]

LGTM. Do we need to do anything special (that hasn't already been done) to indicate that memory64 is not yet compatible with components? Or will that be caught by wasmparser at validation time?

[alexcrichton]

Oh you know that's a good point. I haven't checked up on the state of memory64 and components in awhile. I know some parts support some things but not all, and I suspect that there are a number of memory64-related panics in Wasmtime which I should convert to errors first before landing this.

[alexcrichton]

On review looks like less has support for this than I thought. I've opted to go with a wasmparser validation change for now since components don't currently specify how everything works anyway.