async serial settings interface

[jordens]

• [ ] To address https://github.com/quartiq/stabilizer/issues/885
• [x] Might allow just bubbling up the async-ness of sequential-storage to serial-settings and then higher
• [x] Also make the bounds more consistent here: https://github.com/quartiq/stabilizer/blob/5ea3ee8b62fb56e16b252d758980cac27f7f058f/serial-settings/src/lib.rs#L73-L75
• [x] Use the e-io bytes write, then where needed build a fmt::Write wrapper where needed: https://github.com/quartiq/miniconf/blob/3b4e493e824d5260a5f21fffe67d90a192c625e3/miniconf_cli/src/lib.rs#L15-L33 The latter might be worth upstreaming as well.

[jordens]

Needs async menu. The menu part itself is easy. But async callbacks runs into various issues.