Condvar::wait_while and Condvar::wait_timeout_while convenience methods

[Bryan Hitchcock]

The standard library has Convar::wait_while and Convar::wait_timeout_while methods that take a condition: impl Fn(&mut T) -> bool. These convenience methods are great since the following pattern is relatively common:

while condition(&mut *lock_guard) {
  cv.wait(lock_guard);
}

// condition is now false

I think it'd be great to have these convenience methods built-in, but I can also see an argument being made for not wanting to clutter the public API. Thoughts?

I'm more than happy to submit the PR if this functionality is desired.

[Amanieu d'Antras]

Sure! I'd be happy to accept a PR for this.