Add plain no std example

[kaspar030]

This PR adds a no_std example with working qemu Cortex-M3 target configuration and runner config. Currently it makes only very limited use of edhoc-rs, but should be a good base for more no_std testing.

This was previously at https://github.com/kaspar030/edhoc-rs-no_std. The workspace level .cargo/config.toml previously prevented correct building from inside this (edhoc-rs) repo, which I didn't realize, so I had put it somewhere else. Without that file (which is IMO not needed there), the example can be moved here.

[kaspar030]

• rebased
• changed workspace handling to be in-line with edhoc-crypto-cc2538
• [patch] removed
• added github workflow for building the test. broke my promise about the matrix. :)