Support for positioned io

[g2p]

I'd like to use positioned io (pread / pwrite) deferred to a thread pool. It may be interesting to integrate with one of these:

• https://github.com/vasi/positioned-io
• https://github.com/jmesmon/io-at

[g2p]

My preferred API would take an offset and a length, and return a Stream that concatenates to the exact length.