Add select_timeout for pipes

[eholk]

Part of #1255.

This would take a list of selectables, and return an option of which one was ready (possibly going ahead and doing the receive), or none.

[graydon]

This is not .. exactly done? I think. It's hard to say. I believe the subsystems in question are in the process of being rewritten under the new scheduler, but there is presently a select function in std::comm that calls wait_many. I think this needs @brson to decide if it's live or not.

[bblum]

While we have select, we don't yet have one that won't wait forever.

[emberian]

Visiting for triage; nothing to add

[pnkfelix]

Accepting P-low.

[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#820