C Clubby refactoring

Complete rewrite of the C Clubby implementation: it is now modeled after
the Go implementation, with a slight difference that codec and channel
are the same object (something we should probably do in Go as well, as
we only have a single type of channel so far, which is used with all
types of codecs).

This implementation also comes with a new external API,
which is hopefully cleaner and easier to use (see mg_clubby.h).

In this PR I am not adding any new types of channels, but a UART channel
as well as websocket listener channel will be added after this lands.

PUBLISHED_FROM=d545d4bb6434e2a02ad159f9e4b64e594a4797e7