cpu.rs: Refactor for modularity

- Break into submodules
  - Move bus into submodule of CPU
  - Keep program and charset rom inside CPU
  - Take only the screen on the external Bus
  - Refactor the disassembler into an instruction definition and the actual "Dis" item

tests/chip8_test_suite.rs

@@ -11,7 +11,6 @@ fn setup_environment() -> (CPU, Bus) {
     cpu.flags = Flags {
         debug: true,
         pause: false,
-        monotonic: Some(10),
         ..Default::default()
     };
     (
@@ -36,10 +35,10 @@ struct SuiteTest {
 fn run_screentest(test: SuiteTest, mut cpu: CPU, mut bus: Bus) {
     // Set the test to run
     bus.write(0x1ffu16, test.test);
-    bus.load_region(Program, test.data).unwrap();
+    cpu.load_program_bytes(test.data).unwrap();
     // The test suite always initiates a keypause on test completion
-    while !(cpu.flags.keypause || cpu.flags.pause) {
-        cpu.multistep(&mut bus, 100).unwrap();
+    while !(cpu.flags.is_paused()) {
+        cpu.multistep(&mut bus, 10).unwrap();
     }
     // Compare the screen to the reference screen buffer
     bus.print_screen().unwrap();

tests/integration.rs

@@ -192,7 +192,7 @@ mod cpu {
 }
 
 mod dis {
-    use chirp::cpu::disassembler::Insn;
+    use chirp::cpu::instruction::Insn;
     use imperative_rs::InstructionSet;
 
     #[test]