It was observed that sl_Send can return SL_EAGAIN, despite the fact that
we only call write_to_socket if it was reported as writeable.

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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.

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A way to serve a single, specific file as opposed to the hairy monster
that is mg_serve_http.

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Use templated index page and SSI calls instead of hard-coded html

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In CCS example projects

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We haven't actually _fixed_ evaluation of precompiled bcode from
non-mmapped memory: we just never free it. For sane use case (do not
evaluate the same file more than once) it should be acceptable.

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