Implement Kernel ROM

[bobbinth]

This PR implements Kernel ROM chiplet within the processor crate. This includes:

• Implementing Kernel ROM chiplet.
• Updating the Decoder to rely on Kernel ROM chiplet to check syscall validity.

Things left for future PRs:

• Updating mdBook docs to describe the new structure of chiplets.
• Implementing additional syscall semantics:
  • Setting fmp to $2^{31}$ when for inside syscalls.
  • Adding trace columns to enable checks preventing syscalls from within kernel context.
• Update aux trace column needed for enforcing correctness of kernel ROM (this includes updates to chiplet bus).
  • Setting boundary constraints for the kernel against which a program was executed.

[vlopes11]

Two preliminary comments

[grjte]

Looks good! I left a few things inline, but I'll approve anyway, as they're all comment-related or minor nits