In order to make the Si5351 configurable add some needed assets,
create the ability to print the frequency with selected digits and add
a channel screen.
Create `print_u8()` helper function to print `u8` primitives to
lcd. Refactor contrast and backlight variables to also keep them in a
global `AtomicU8`.
To reduce length of `main.rs` and therefor improve readability create
source files for:
- Assets (`assets.rs`): Contains all graphical assets such as the
splash screen assets, symbols and the symbol table.
- LCD (`lcd.rs`): Contains all lower level things regarding the LCD
such as the `Lcd` struct and its implementations.
- Screen (`screen/mod.rs`):
- Splash (`screen/splash.rs`)
- Home (`screen/home.rs`)
- Setup (`screen/setup.rs`)
In the future it would probably make sense to move the LCD module into
the screen module.
