add limited timed wait to semaphore

[drewcrawford]

I think Semaphore could use timeout variants of acquire and access, besides just blocking forever.

The underlying Condvar already has timeout support, so it's just a matter of bubbling it up to semaphore. Looking at sempahore.rs, it seems like low-hanging fruit to add.

I think the biggest question is whether it should be called acquire_timeout and access_timeout (following Condvar's wait_timeout) or whether it should be called try_acquire and try_access (following Mutex and RwLock).

If there's a decision rendered on the syntax, this would be a good feature for a new contributor.

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#853