CLN: remove items/ref_items attributes from Block class

Now, there's one authoritative mapping from BlockManager axis0 to
axis0 of subordinate blocks:

* _blknos[loc] is block number where loc-th item resides
* _blklocs[loc] is location in given block of loc-th item

Blocks support this mapping with blk._ref_locs attributes that
enumerate all items contained in that block.
CLN: rename Block.ref_locs -> mgr_locs to reduce confusion

PERF: add BlockPlacement class to optimize range-like cases

CLN: unify get_slice & take/reindex ops along axis0

TST: add slicing/taking/reindexing tests for BlockManagers

CLN: remove unused code
Merge pull request #6745 from immerrr/refactor-blockmanager

CLN: revisit & simplify Block/BlockManager, remove axes