Now that JS `Atomics.wait` natively supports BigInt64Array,
`memory.atomic.wait64` can be described directly in terms of `Atomic.wait`.

Since #144, we've specified that `memory.atomic.notify` is allowed to be executed on unshared memories, but always returns 0 since it is impossible for there to be anything waiting on an unshared memory since all variations of the wait instructions in Wasm and JS trap or throw exceptions on unshared memories. However, I realized recently that there is no technical reason why [`Atomics.waitAsync`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Atomics/waitAsync) shouldn't be able to wait on an unshared memory. Because `Atomics.waitAsync` returns a promise rather than blocking, it can in principle be used to synchronize between multiple asynchronous tasks in flight concurrently in the same JS context, and now that [JSPI](https://github.com/WebAssembly/js-promise-integration/blob/main/proposals/js-promise-integration/Overview.md) is becoming more real, this actually might be a useful thing to do. In particular `Atomics.waitAsync` could be used to build suspending versions of mutexes and condition variables to synchronize between threads running as different asynchronous tasks within a single JS context.

Actually realizing this vision of asynchronous threading with JSPI on top of `Atomics.waitAsync` requires changes on the JS side to allow `Atomics.waitAsync` and `Atomics.notify` to be used on unshared ArrayBuffers, but in the meantime we can prepare for that change on the Wasm side by softening the language stating that `memory.atomic.notify` unconditionally returns 0 for unshared memories.

…#192)

* implement atomic.fence, add wait/notify edge case tests

* Update interpreter/exec/eval.ml

Co-authored-by: Andreas Rossberg <rossberg@dfinity.org>

While implementing wasm-threading in JavaScriptCore, we found that this edge cases are not tested in spec-tests.
To clarify cmpxchg behavior, this patch adds them to spec-tests.

* implement wait and notify

Co-authored-by: Andreas Rossberg <rossberg@dfinity.org>

Prior to this commit there was a copy/paste typo in the Compare Exchange table wherein a 16-bit operation is said in one column to be 8-bit (pasted from the line above).

This commit fixes the typo so that it reflects the correct number of bits.