Add a feature for logging panics with defmt

[TheButlah]

In line with the print-uart and print-rtt features, perhaps a print-defmt would make sense?

I am happy to submit a PR for this, however I have a question. The current implementation calls several println!() functions. In most defmt log printers, the contents of the log statement gets printed along with the location in the source code that the log statement was called.

Would this mean that we would have only one error!() invocation in the panic_handler? Or should we still call error!() each time we call println().

Also, due to this quirk of defmt vs println style printing, should I add this functionality to esp-println? Because it may be a mismatch of expected output format.

[bjoernQ]

I'm not too familiar with defmt to be honest. Probably error!() is more like dbg! from the std-lib, then?
Sounds like you probably only want one invocation of error! in your implementation

By "should I add this functionality to esp-println" you mean printing the source code location? I don't think we should do that for println but if you like you can add a dbg macro just like https://github.com/rust-lang/rust/blob/efa717bc2d82c19c6957f362a936a6f95169d138/library/std/src/macros.rs#L339-L361 to esp-println

Otherwise, happy to receive a PR for this

[TheButlah]

By "should I add this functionality to esp-println" you mean printing the source code location?

No, I meant should I add a print-defmt feature to esp-println. I'm not sure if its a good match because defmt introduces its own formatting by attaching line numbers and log levels to each print. See this example:

[jessebraham]

I'm just starting to learn defmt myself, but esp-println at least has support for it now. Maybe somebody can tackle this issue.

[bjoernQ]

I guess this is covered by #43