chore: For WASI builds only, use fatalError in all .wait() calls. Recommend using .get() instead.

[scottmarchant]

Use fatalError in all .wait() calls for WASI builds only, and point developers towards .get() instead.

Motivation:

While working to adopt NIO in some wasm code, I've commonly ran into issues any time code calls .wait() during wasm runtime. Typically the executable either traps or crashes any time .wait() is called with an ambiguous error: Uncaught (in promise) RuntimeError: Atomics.wait cannot be called in this context. The error occurs because it is forbidden to block the main thread for a wasm executable, and the current implementation of .wait() blocks the calling thread.

The fix is straight forward, all calls to .wait() need refactor to .get(), which sometimes involves some Swift Concurrency adoption (eg. async) in the process. That change avoids blocking the main thread.

Modifications:

Added fatalError with a descriptive error message to help developers identify the issue easier.

Result:

For WASI builds only, changes the trap error message Uncaught (in promise) RuntimeError: Atomics.wait cannot be called in this context into the following error message instead:

NIO's wait() function should not be called on WASI platforms. It will freeze or crash. Use get() instead.

Alternatives considered

• We could instead conditionalize .wait() completely out of nio for WASI builds, to turn runtime errors into compiler errors. That makes detection of this issue easier, but forces a refactor and breaking change, and somewhat precludes the possibility that WASI might support this down the road with a future change.
• We could add a deprecation for WASI only. But long term WASI may be able to support this blocking call, so deprecation and removal is communicating the wrong message if the future ends up being reality.

Context

This PR is part of a larger effort by PassiveLogic to move Swift for WebAssembly support forward in a large number of dependencies.

[scottmarchant]

@MaxDesiatov @kateinoigakukun Would love to have your thoughts on this. Open to other solutions. But from what I've experienced so far, having this change could be very helpful for developers newly adopting swift nio into their WASI builds.

[scottmarchant]

Rebased these changes on the latest main