introduce iox package: I/O interface exchange utilities.
add writer conversions to iox package
drop concept of writers to byte slice from iox.  it's just too weird.
gofmt
implement sh.BakeOpts, and drag in the iox package, so you can now set stdin, stdout, and stderr to whatever of a half-dozen forms you please.
create predefined psh.DefaultIO that can be easily baked into a command to attach the in/out/err channels from the parent process.
demo hooking up a string as input pipe.  so easy.