[wip] incremental swizzle implementation

[spacejam]

This is an incremental new approach of pointer swizzling, replacing #1300 which proved to be a bad approach that caused things to get bogged down. This PR will implement similar things over time, but with an eye toward keeping each commit more-or-less functional instead of the shotgun approach that didn't pan out in the previous attempt.

• [ ] first priority: paged-out single frag log items stored in a single pointer

  • [x] fuzzy snapshot
  • [ ] trim snapshot
  • [ ] make HeapId single-word
    • [ ] remove LSN from HeapId
  • [ ] remove log pointers for heap items during snapshot creation
  • [ ] make DiskPtr single-word in standard case, moving to a single allocation while it's log + heap
  • [ ] add discriminant to low bits of pagetable entries
  • [x] remove CacheInfo dependencies from segment tracker
  • [ ] page-in fragmented pages on startup

• [ ] secondary goals

  • [ ] use swizzle discriminant + log coordinate for free entry not yet merged into snapshot
  • [ ] use null for free pages that have been merged into the last snapshot
  • [ ] collapse DiskPtr into inline enum with swizzled boxed coords for log+heap
  • [ ] make disk info inline with pages and frags to avoid the vec allocation
  • [ ] let frags replace single frags for the same key to avoid merges on hot keys
  • [ ] remove update enum from page representation

non-goals for this pr:

• intermediate node structs are ok for now