CLN: revisit & simplify Block/BlockManager, remove axes

This is a WIP request for Stage 1 deliverable of #6744.

Roadmap:
   - [x] make ref_locs primary source of information (leaving items/ref_items in place to back it up and avoid breakage)
   - [x] port groupby  to loc-based implementation (there's quite a number of hacks a.t.m. that make this non-trivial)
   - [x] ditto for merge/join/concat
   - [x] ditto for io code
   - [x] ditto for reshape
   - [x] clean up Block `items`
   - [x] clean up Block `ref_items`
   - [x] fix performance issues & integrate with mainline