Changes:

* Update to coap-message 0.3 and its fallout updates.

* Replace never_type with an own Never enum.

... because all dependencies are now usable on stable.

Changes:

* Fix closing of sockets

Previously, sockets were only closed on aborts, but not when errors
propagated out.

Documentation was updated to reflect that; the previous requirement that
the user needs to close the connection was already impossible to
fullfil.

Changes:

* Disable unused default features to be really no-std

The socket can't be used for anything outside anyway (that'd just be
calling for protocol errors), and this allows closing it right at abort
time.

The first-byte writes can just be writes rather than or-ing, and then
all that's used is written to as well.

... and other clippy suggestions

Doesn't change much now but makes it easier to handle out-of-send-buffer
more gracefully.

This is *not* making best use of send_all, primarily due to choices of
this implementation. (With other choices, the options would be better).

See-Also: https://github.com/rust-embedded-community/embedded-nal/issues/59

anticipating there might also soon be a SENDBUFLEN

... not that that'd guarantee that it all goes through, which is an
issue yet to be tackled.