Fix the organization of the Selectable trait, etc.

[brson]

comm is the user-facing interface to pipes. It currently reexports pipes::Selectable so it can be used by selecti and select2i. This exposes a single detail of core::pipes, which otherwise users don't need to be aware of. Decide whether this trait belongs to pipes or to comm.

[pnkfelix]

This should probably be folded into Issue #6842.

[pnkfelix]

(but I won't close it; instead it will remain as a thorn in the side of whomever is implementing #6842. :)

[bblum]

The same issue exists in the new implementation of select currently in libstd/rt/select.rs. We discussed a plan for fixing the trait definitions in #8008.

[graydon]

Appears to be still relevant. Properly classified, moving on.

[bblum]

Also, the select routine itself can be moved out of the runtime.