Rework Pipe to create (xio.Reader, xio.Writer) instead of (io.Reader,
io.Writer) and teach xio.Pipe{Reader,Writer} to accept ctx argument and
handle cancellation.

I need this to support sysread(/head/watch) cancellation in WCFS
filesystem [1,2,3]. See also [4].

[1] wendelin.core@b17aeb8c
[2] wendelin.core@f05271b1
[3] wendelin.core@5ba816da
[4] https://github.com/golang/go/issues/20280