Make Readable::read a templated on the stream, not Readable itself

Make Readable::read a templated on the stream, not Readable itself

This makes Readable symmetric with Writeable and makes sense -
something which is Readable should be Readable for any stream which
implements std::io::Read, not only for a stream type it decides on.

This solves some lifetime-compatibility issues in trying to read()
from a LengthLimitingReader in arbitrary Readable impls.