add defmt::panic!

[japaric]

closes #136
this implements defmt::panic! which calls defmt::error! and then "panics"

by default, defmt::panic! will "panic" by calling core::panic!. This preserves the panicking behavior (e.g. abort on panic, reset on panic, etc.) but can result in the panic message being printed twice -- this is the case if you use panic_probe (+print-defmt)

to prevent printing the panic message twice, the panicking behavior of defmt::panic can be overridden using the #[defmt::panic_handler] attribute, which works about the same as #[core::panic_handler] but expects a fn() -> ! function (note: no argument)

the idea is that end users will replicate their #[panic_handler] panicking behavior, and omit the 'print the panic message' part, in the #[defmt::panic_handler] function, for example:

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    print(info); // e.g. using defmt
    reset()
}

#[defmt::panic_handler]
fn defmt_panic() -> ! {
    reset()
}

we can do the above for all new users of app-template so they don't run into the 'panic message printed twice':

// src/lib.rs
use panic_probe as _; // "print-defmt" is enabled

#[defmt::panic_handler]
fn panic() -> ! {
    cortex_m::asm::udf()
}

TODO

• API docs
• book docs
• todo! & unimplemented!
• unreachable!
• assert!
• assert_eq! assert_ne!
• debug_assert, debug_assert_eq, debug_assert_ne
• snapshot tests
• assert_eq! & probe-run integration (colored diff) -> https://github.com/knurling-rs/probe-run/tree/color-diff
• prelude didn't work you can't override core's prelude items using a glob import

FIXME

• probe-run renders assert_ne as a color-diff but shouldn't

[jonas-schievink]

Great!

[jonas-schievink]

cc #252