Swap return values of pipes::init()

[nickdesaulniers]

This is a continuation of sorts of Issue #4496. In that issue, the return values of the tuple returned from calling pipes::init() were changes from (channel, port) to (port, channel) to better conform with other calling conventions elsewhere in the code base. I noticed that pipes::init() also went against convention, but don't yet have the understanding of self hosting compiler issues to tackle this just yet. A read through Issue #4496 is recommended.
ack --type-add=rust=.rs,.rc oneshot::init

[nickdesaulniers]

Assign=me

[nikomatsakis]

Not critical for 0.6; removing milestone

[nickdesaulniers]

Looks like the definition of oneshot::init was moved.