fw-rust: Break everything down in multiple files

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.

firmware/rust/src/screen/home.rs

@@ -0,0 +1,83 @@
+use super::{Screen, Screens, Setup, Splash};
+use crate::{lcd::Lcd, Input};
+
+enum Selection {
+    Ch1,
+    Ch2,
+    Ch3,
+    Setup,
+}
+
+pub struct Home {
+    active: Selection,
+}
+
+impl Home {
+    pub fn new() -> Self {
+        Self {
+            active: Selection::Ch1,
+        }
+    }
+
+    pub fn input(&self, input: &Input) -> Screens {
+        Screens::Home(Self {
+            active: match self.active {
+                Selection::Ch1 => match input {
+                    Input::Next => Selection::Ch2,
+                    Input::Previous => Selection::Setup,
+                    Input::Select => return Screens::Splash(Splash),
+                    Input::Back => Selection::Ch1,
+                },
+                Selection::Ch2 => match input {
+                    Input::Next => Selection::Ch3,
+                    Input::Previous => Selection::Ch1,
+                    Input::Select => return Screens::Splash(Splash),
+                    Input::Back => Selection::Ch2,
+                },
+                Selection::Ch3 => match input {
+                    Input::Next => Selection::Setup,
+                    Input::Previous => Selection::Ch2,
+                    Input::Select => return Screens::Splash(Splash),
+                    Input::Back => Selection::Ch3,
+                },
+                Selection::Setup => match input {
+                    Input::Next => Selection::Ch1,
+                    Input::Previous => Selection::Ch3,
+                    Input::Select => return Screens::Setup(Setup::new()),
+                    Input::Back => Selection::Setup,
+                },
+            },
+        })
+    }
+}
+
+impl Screen for Home {
+    fn draw(&self, lcd: &mut Lcd) {
+        match &self.active {
+            Selection::Ch1 => {
+                lcd.print_inverted(0, 0, "CH1");
+                lcd.print(0, 2, "CH2");
+                lcd.print(0, 4, "CH3");
+                lcd.print(33, 6, "SETUP");
+            }
+            Selection::Ch2 => {
+                lcd.print(0, 0, "CH1");
+                lcd.print_inverted(0, 2, "CH2");
+                lcd.print(0, 4, "CH3");
+                lcd.print(33, 6, "SETUP");
+            }
+            Selection::Ch3 => {
+                lcd.print(0, 0, "CH1");
+                lcd.print(0, 2, "CH2");
+                lcd.print_inverted(0, 4, "CH3");
+                lcd.print(33, 6, "SETUP");
+            }
+            Selection::Setup => {
+                lcd.print(0, 0, "CH1");
+                lcd.print(0, 2, "CH2");
+                lcd.print(0, 4, "CH3");
+                lcd.print_inverted(33, 6, "SETUP");
+            }
+        }
+    }
+}