tests: Coverage and cleanup/speedup

- Improved test coverage to >80% of lines, functions
  - When doctests are included.
  - Wrote new unit tests:
    - Explicit tests for invalid instructions in the
      ranges {`5xyn`, `8xyn`, `9xyn`, `Fxbb`}
    - `rand` Tests for 1052671 cycles, to ensure
      randomly generated number is < ANDed byte
    - `Ex9E` (sek), `ExA1`(snek) will press only the expected key,
      then every key except the expected key, for every address
    - `Fx0A` (waitk) asserts based on the waveform of a keypress.
      After all, an A press is an A press.
- Improved test performance by printing slightly less
- Removed nightly requirement
  - (now optional, with feature = "unstable")
- Amended justfile to test with `cargo nextest` (nice)
- Changed release builds to optlevel 3

src/cpu.rs

@@ -74,10 +74,35 @@ pub struct ControlFlags {
 
 impl ControlFlags {
     /// Toggles debug mode
+    ///
+    /// # Examples
+    /// ```rust
+    ///# use chirp::prelude::*;
+    ///# fn main() -> Result<()> {
+    ///     let mut cpu = CPU::default();
+    ///     assert_eq!(true, cpu.flags.debug);
+    ///     cpu.flags.debug();
+    ///     assert_eq!(false, cpu.flags.debug);
+    ///#    Ok(())
+    ///# }
+    /// ```
     pub fn debug(&mut self) {
         self.debug = !self.debug
     }
+
     /// Toggles pause
+    ///
+    /// # Examples
+    /// ```rust
+    ///# use chirp::prelude::*;
+    ///# fn main() -> Result<()> {
+    ///     let mut cpu = CPU::default();
+    ///     assert_eq!(false, cpu.flags.pause);
+    ///     cpu.flags.pause();
+    ///     assert_eq!(true, cpu.flags.pause);
+    ///#    Ok(())
+    ///# }
+    /// ```
     pub fn pause(&mut self) {
         self.pause = !self.pause
     }