wiggle: add initial support for shared memory (#5225)

This change is the first in a series of changes to support shared memory
in Wiggle. Since Wiggle was written under the assumption of
single-threaded guest-side access, this change introduces a `shared`
field to guest memories in order to flag when this assumption will not
be the case. This change always sets `shared` to `false`; once a few
more pieces are in place, `shared` will be set dynamically when a shared
memory is detected, e.g., in a change like #5054.

Using the `shared` field, we can now decide to load Wiggle values
differently under the new assumptions. This change  makes the guest
`T::read` and `T::write` calls into `Relaxed` atomic loads and stores in
order to maintain WebAssembly's expected memory consistency guarantees.
We choose Rust's `Relaxed` here to match the `Unordered` memory
consistency described in the [memory model] section of the ECMA spec.
These relaxed accesses are done unconditionally, since we theorize that
the performance benefit of an additional branch vs a relaxed load is
not much.

[memory model]: https://tc39.es/ecma262/multipage/memory-model.html#sec-memory-model

Since 128-bit scalar types do not have `Atomic*` equivalents, we remove
their `T::read` and `T::write` implementations here. They are unused by
any WASI implementations in the project.

crates/wiggle/generate/src/wasmtime.rs

@@ -118,7 +118,7 @@ fn generate_func(
         };
         let (mem , ctx) = mem.data_and_store_mut(&mut caller);
         let ctx = get_cx(ctx);
-        let mem = #rt::wasmtime::WasmtimeGuestMemory::new(mem);
+        let mem = #rt::wasmtime::WasmtimeGuestMemory::new(mem, false);
         Ok(<#ret_ty>::from(#abi_func(ctx, &mem #(, #arg_names)*) #await_ ?))
     };