Add SEGGER RTT interface using `rtt-target` crate

[berkus]

See https://github.com/metta-systems/vesper/tree/explore/jlink-rtt

• postponed in feature/add-rtt branch, current situation with RTT support on the host side is pretty bad.

[berkus]

Use defmt-rtt instead perhaps, as rtt-target is a bit frivolous with structures accesses.