Instruction handling & Memory improvements

[sameer-s]

• [ ] Make instruction memory non-writeable.
• [ ] Pre-parse instructions into an internal data structure besides Memory.
  • [ ] Explore ability to combine frequent instruction sequences into higher-level pseudo-instructions that may be executed directly.
• [ ] Switch memory to a sparse representation. Current page-based randomization approach incurs unnecessary overhead, since it is only relevant when reading uninitialized values, which we hope should be a rare occurrence.

[ammrat13]

Do we want to make instruction memory non-writable. IIRC the book allows writes to instructions that are about to execute.