Add an "off-the-shelf" debugging panic handler

[Rahix]

During development, a panic handler that actually shows the panic message is very important. In practice, this usually means printing the message to serial/UART, as demonstrated in this example: uno-panic.rs

It would be nice to include a sane-default implementation of such a panic handler somewhere in our crates.

Some thoughts:

• As a first step, just adding it to arduino-hal would serve most users.
• It should be opt-in via a crate feature flag, so people can replace it with their own handler easily.
• It should use the default serial device, as "specified" by arduino_hal::default_serial!()
• It should not modify the serial settings that were configured by the user. I think the best solution would be to initialize the UART peripheral without changing any of the settings (needs additional support in avr-hal-generic).
• At some point in the future, integration with the USB-serial for ATmega32U4, etc. should be considered...