Split core::pipes into core::comm and core::rt::pipes

[brson]

The end-user parts, stream, Chan, Port, etc. become the new comm. The rest can hide with other runtime code (#2240) in core::rt::pipes. We maybe don't want the implementation details to have a stable interface yet.

[brson]

core::rt is priv so at least some of the pipe module would still need to be reexported from core::private, or whatever it gets called (#4743).

[brson]

I don't think we need to move pipes to core::rt after all. Instead make it private and reexport the compiler interface under core::internal (or private or whatever it turns out to be).

[brson]

Done.