NFC: tidy up the stock GC codebase (part 1) (#60278)
Non-functional changes to improve the readability of the stock GC code:
- Add comments and ASCII diagrams documenting the layout of
pool-allocated pages and the multi-level page table containing page
allocation status.
- Replace `static inline` with `STATIC_INLINE` for consistency with the
rest of the GC.
- Move or remove a few functions for better file encapsulation and
organization.
- Delete unused variables from GC headers.
- Replace nested conditional statements with guarded clauses to improve
readability.
