Implement memory management unit for allocation and access.

[yezzfusl]

1. A new memory.rs file that implements the MemoryManagementUnit struct.
2. The MemoryManagementUnit provides methods for reading and writing both bytes and words (32-bit values) to memory.
3. The CPU struct now uses the MemoryManagementUnit instead of a raw memory array.
4. Two new instructions, load and store, have been added to interact with memory.
5. The fetch method now uses the MMU to read instructions from memory.
6. Additional tests have been added to verify the correctness of memory operations and the new instructions.

[yezzfusl]

The memory management unit is implemented efficiently. Memory allocation, reading, and writing operations are handled correctly. The code is clean and modular, making it easy to follow.