Expand description
A trait for the combination of reading and seeking.
TODO(JP): BufReader is explicitly not included here since its Seek
behavior sucks: it causes clearing its internal buffer. Instead you have
to use BufReader::seek_relative, but that breaks this nice transparent abstraction,
so maybe we should make our own BufReader variant that doesn’t clear
on seek, and can be transparently passed into anything that accepts
ReadSeek.