Refactor LengthPrefixed{De, En}coder to Frame{De,En}coder.

Tests are not fixed yet.

Improve decoding error messages by annotating them with the position
at which the error was encountered. This could use some improvement
in a follow-up by introducing a specific Error type instead of
piggy-backing onto std::io::Error.

It is easier and more generically useful to accept any byte slice
as a buffer. In addition our zero-copy use of the underlying buffer
does not align particularly well with std::io::Read, so we move away
from std::io::Cursor as well.

First step towards hiding ProtoEncode as an implementation detail and
only exposing Encode externally.