Add functional test and update CPU debug output

The functional test is based on
https://github.com/Klaus2m5/6502_65C02_functional_tests

It does NOT pass yet. :(
However, I think it's a good idea to have it in the repo
nonetheless, to make some incremental improvements.

This PR is based on the `asm` branch, which should be
merged first.

examples/asm/functional_test/README.md

@@ -0,0 +1 @@
+From https://github.com/Klaus2m5/6502_65C02_functional_tests

examples/functional.rs

@@ -0,0 +1,34 @@
+use mos6502::cpu;
+use mos6502::memory::Bus;
+use mos6502::memory::Memory;
+use std::fs::read;
+
+fn main() {
+    // Load the binary file from disk
+    let program = match read("examples/asm/functional_test/6502_functional_test.bin") {
+        Ok(data) => data,
+        Err(err) => {
+            println!("Error reading functional test: {}", err);
+            return;
+        }
+    };
+
+    let mut cpu = cpu::CPU::new(Memory::new());
+
+    cpu.memory.set_bytes(0x00, &program);
+    cpu.registers.program_counter = 0x400;
+
+    // run step-by-step
+    let mut old_pc = cpu.registers.program_counter;
+    while cpu.registers.program_counter != 0x3468 {
+        cpu.single_step();
+        println!("{cpu:?}");
+
+        if cpu.registers.program_counter == old_pc {
+            println!("Infinite loop detected!");
+            break;
+        }
+
+        old_pc = cpu.registers.program_counter;
+    }
+}

src/cpu.rs

@@ -988,11 +988,7 @@ impl<M: Bus> CPU<M> {
 
 impl<M: Bus> core::fmt::Debug for CPU<M> {
     fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
-        write!(
-            f,
-            "CPU Dump:\n\nAccumulator: {}",
-            self.registers.accumulator
-        )
+        write!(f, "CPU {{ registers: {:?}", self.registers)
     }
 }