defmt code size testbench

Small repo to measure defmt code size improvements. Contains 2 benchmarks

• spam: prints lots of random things. based on this.
• smoltcp: runs the smoltcp loopback example.

How to use:

• Clone defmt and probe-run next to this repo
• Patch probe-run to get defmt-decoder from ../defmt/decoder
• cargo run --release --bin spam (or smoltcp)
• cargo size --release --bin spam

To test PR 258 edit rtt.rs and uncomment rtt_2_nowrite