Merge: Read byte rewrite

As part of the stream refactor series, this patches cleanup msgpack a bit and introduce a lower-level `read_byte` implementation on which `read_byte(s)` will rely for reading data from a source.

In the future, the lowest-level interface to read bytes, and the only one required for implementing a new stream will be the `raw_read_byte(s)` functions.

A cleanup of the current streams will be needed for this to be used properly, coming in the next few PRs.

Note: The first two commits have already been submitted under #2632, therefore disregard them for this PR.

Pull-Request: #2633
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>