CLN: revisit & simplify Block/BlockManager, remove axes

This is a WIP request for Stage 1 deliverable of #6744.

Roadmap:
   - [ ] make ref_locs primary source of information (leaving items/ref_items in place to back it up and avoid breakage)
   - [ ] port merging/joining internals to loc-based implementation (there's quite a number of hacks a.t.m. that make this non-trivial)
   - [ ] drop Block `items` & `ref_items` fields (ensure io backward compatibility!)
   - [ ] fix performance issues & integrate with mainline