async serial settings interface

[jordens]

• To address usb serial terminal buffer issue #885
• Might allow just bubbling up the async-ness of sequential-storage to serial-settings and then higher
• Also make the bounds more consistent here:
  

  stabilizer/serial-settings/src/lib.rs

  Lines 73 to 75 in 5ea3ee8

  	type Interface: embedded_io::Read
  	+ embedded_io::ReadReady
  	+ core::fmt::Write;

• 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.