Add defmt logging support

[Dirbaio]

This adds basic support for optionally logging with defmt instead of log, controlled via Cargo features.

Any struct that can implement defmt::Format now does so. There's a few left, due to:

• Missing impls for Cell, RefCell that should be added in defmt.
• Missing impls for managed types.

The left structs are not used in log statements anyway, and are usually "big" structs, like sockets. Since this is already fully functional for integration in a firmware that uses defmt I would merge this now, and add the missing impls later when they're unblocked.

Fixes #376

[therealprof]

Hm, what's the point of these, are they needed by defmt?

[Dirbaio]

Yes, the defmt macros only emit code if the corresponding log level (or lower) is enabled. The end-user is supposed to depend on smoltcp with defmt, defmt-debug features to see up to debug level, for example. This is so log level is configurable per-crate.

[therealprof]

But don't you need to forward the features then? I'm confused about how this would work as-is in this PR.

[therealprof]

Sweet! Any comparison available between typical binary size and defmtenabled?

[Dirbaio]

defmt won't build with 1.40, which is our MSRV. The lowest I can get this to build is with 1.46, see knurling-rs/defmt#443

I guess the best option is to declare our MSRV is 1.40 except if you're using defmt, in which case the limiting factor is defmt's MSRV.

[Dirbaio]

Any comparison available between typical binary size and defmtenabled?

Unfortunately no :( I've been using this internally for a while already and no longer have a firmware that uses log handy...