feat(maitake): add `WaitCell::subscribe` and `poll_wait`

[hawkw]

This branch rewrites the maitake::sync::WaitCell type a bit to fix potential lost wakeups accidentally introduced in #453. In addition, it changes the API a bit:

• Replace register_wait with poll_wait, which I think is a bit nicer,
• Removed the RegisterError::Waking variant since poll_wait returns Ready if the cell was woken, and renamed RegisterError to just Error,
• Introduced a WaitCell::subscribe method to eagerly pre-register the task's Waker in the WaitCell. This is intended to be used to register the task to be woken before performing an operation that results in a wakeup, such as enabling an interrupt that will wake the task.

Closes #455

[hawkw]

okay, you know what? this PR was the worst thing that's ever happened to me. i'm going to give up on trying to write a test to reproduce the last problem.