Bump all edition to 2021, run `cargo update` and manually bump the
following dependencies:
- `nb`
- `avr-device`
Fix several and meanwhile unnecessary uses of unsave blocks and a
unneeded cast.
Add several symbols and a symbol table in an ascii order. Add the
`print()` and `print_inverted()` functions to be able to draw strings,
"black on white" as well as "white on black" onto the display. Add a
`HomeScreen` which also saves the state of which selection is
active. To save the screen state and also make it accessible globally
use a `RefCell` contained in a `Mutex`.
Also update the build dependencies.
