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.
There's quite a bit of machinery included here, but it neatly
avoids any dynamic allocation during TLV deserialization, and the
calling side looks nice and simple. The macro-generated code is
pretty nice, though has some redundant if statements (I haven't
checked if they get optimized out yet, but I can't imagine they
don't).
