Implement state saving and loading for CPU and Bus, add tests for various functionalities

- Added `state.c` to handle saving and loading the state of the CPU and Bus.
- Implemented CRC32 checksum for data integrity verification during state operations.
- Enhanced `README.md` in tests/roms to document new test commands for decimal, interrupt, Apple I, and nestest.
- Created new test files: `run_apple1.c`, `run_decimal.c`, `run_interrupt.c`, `run_nestest.c`, and `run_vectors.c` to validate the new features.
- Updated `run_tests.c` to include tests for state saving/loading and other functionalities.
- Introduced `test_vectors.py` to facilitate downloading and running JSON test vectors for opcode verification.

include/cpu.h

@@ -13,6 +13,7 @@ typedef struct CPU6502 {
     uint8_t P;
     bool nmi_pending;
     bool irq_asserted;
+    bool decimal_enabled;
 } CPU6502;
 
 enum {