rust: Add support for inter-task wakeups

[alexcrichton]

This commit fixes a preexisting issue in the async support in the rust
bindings where inter-component-model-task wakeups/communication did not
work. Previously wit-bindgen would end up either panicking or wedging
itself if this situation arose, effectively not handling it at all.

Inter-task wakeups are supported in the component model with unit
streams and futures (e.g. future and stream with no type parameter).
For these types the component model allows the same instance to both
read/write from the stream or future. The support here is implemented
with stream as a task might receive many wake up notifications over
its lifetime.

Implementing this is a bit tricky, so the highlights of the
implementation are:

• This relies on new "magic function imports" recognized by
  wit-component specifically related to unit-streams and unit-futures.
  This is a very new feature of wit-component and thus hasn't had time
  to propagate throughout the ecosystem yet. Due to the niche nature of
  needing cross-task wakeups this commit disables cross-task wakeups by
  default and adds support for it behind a Cargo feature. The thinking
  is that if anyone runs into this there's now a clear panic/error
  message indicating that the feature can be enabled. Otherwise the hope
  is that no one runs into this (as it's relatively niche) and when the
  time is right we can enable this by default.

• All tasks started by wit-bindgen will need this new inter-task-wakeup
  stream for the reason that we don't actually know what will wake a
  task up when it blocks. To avoid adding a stream allocation on the
  fast path, the allocation of the inter-task stream is deferred until
  a task blocks for the first time.

• Right now a read of the inter-task stream starts when a task blocks.
  The read is cancelled when the task wakes up. A future optimization
  would be to keep the read-in-progress and only restart it if
  necessary. For now that's deferred, however.

Closes #1431

[alexcrichton]

This is temporarily built on #1439 and relies on unpublished versions of wasm-tools so CI will be broken.