Add no_std support to binrw::io::BufReader

Currently implemented as a wrapper type this could either be a no_std reimplementation or just a wrapper around whichever implementation is available.

Start by reading the binrw::io::BufReader implementation and how its current wrapper type depends on standard-library I/O. Compare the available no_std approach with wrapping an implementation available in that environment. Done means BufReader supports no_std without breaking its existing behavior.

Written by the indexing model from the issue text.