In many cases IO needs to be able to be canceled. For example in WCFS
filesystem I need to cancel handling sysread(/head/watch) when FUSE
INTERRUPT request comes in [1,2,3]. The READ handler for /head/watch
inside WCFS is interally implemented via io.Pipe which does not provide
read/write cancellattion besides "destructive" close.

Standard Go answer for cancellations is via contexts. So as a first step
let's add corresponding interfaces - xio.Reader, xio.Writer etc - that
are io analogs that add support for contexts.

For compatibility with legacy code that work with only io.X (e.g.  only
with io.Reader), in spirit of [4], add BindCtx which binds xio.X
instance with context and converts it into io.X. Add WithCtx -
corresponding inverse operation that converts io.X back into xio.X and
for general io.X adds cancellation handling on a best-effort basis.

[1] wendelin.core@b17aeb8c
[2] wendelin.core@f05271b1
[3] wendelin.core@5ba816da
[4] https://github.com/golang/go/issues/20280
[5] https://github.com/golang/go/issues/16522