Refactor futures/streams support for Rust (#1249)

* Refactor futures/streams support for Rust

Mostly remove the old implementations for futures/streams and replace
them with new versions which address a number of preexisting issues and
additionally adapt to upcoming ABI changes.

* Don't run new tests just yet

* Go back to original `wac` deps

* Disable patch overrides

* Beef up some documentation in various places

* Remove some dead code

* Defer async tests to a future PR

* Add some tests to `AbiBuffer<T>`

* Try to fix CI

* Add miri too

* Add some built-in debugging support

Require source edits for now though to enable it.

* Document `CompletionStatus`

* Review comments

Author: alexcrichton

.github/workflows/main.yml

@@ -133,6 +133,11 @@ jobs:
     - run: cargo test -p wit-bindgen-core
     - run: cargo test -p wit-bindgen
     - run: cargo test --workspace --exclude 'wit-bindgen*'
+    - run: cargo test -p wit-bindgen-rt --all-features
+    - run: rustup update nightly --no-self-update
+    - run: rustup component add miri --toolchain nightly
+    - run: rustup component add rust-src --toolchain nightly
+    - run: cargo +nightly miri test -p wit-bindgen-rt --all-features
 
   check:
     name: Check