Make the pipe code take an interface as the backing store.  Now a pipe
is something that's goroutine-safe and does the Cond waits but its underlying data
is now an interface: anything that's a ReaderWriter with a Len method (such as a
*bytes.Buffer), or a fixedBuffer (renamed in this CL from 'buffer').

This opens the ground to having a non-fixed buffer used with pipe.

This also moves the CloseWithError code up into the pipe code, out of
fixedBuffer.

Change-Id: Ia3b853e8aa8920807b705ff4e41bed934a8c67b7
Reviewed-on: https://go-review.googlesource.com/16312Reviewed-by: Blake Mizerany <blake.mizerany@gmail.com>