Make the select! macro work with newrt pipes.

[bblum]

Old pipes used a common RecvPacketBuffered struct that all types of pipes shared in common to enable heterogeneous select. I've made a version that uses traits for new runtime ports, as can be seen in libstd/rt/select.rs. (But a &Trait bug prevents the heterogeneous version from working currently.)

Anyway, I have no idea how the old select! macro translated its input to something that used the select function. But the select2 function I added demonstrates how you could create a list of arbitrary length of heterogeneous selectables, and select on them, so we ought to be able to port the select! macro to it.

related #5160 #6842

nominating feature-complete

[catamorphism]

Just a bug

[alexcrichton]

Closing for being outdated. std::comm has now been rewritten and the select! macro has different constraints now.