complete btree rewrite

Replaces BTree with BTreeMap and BTreeSet, which are completely new implementations.
BTreeMap's internal Node representation is particularly inefficient at the moment to
make this first implementation easy to reason about and fairly safe. Both collections
are also currently missing some of the tooling specific to sorted collections, which
is planned as future work pending reform of these APIs. General implementation issues
are discussed with TODOs internally

[breaking-change]

Still waiting on compilation/test/bench stuff locally, but the edit-distance on any errors should be very small at this point. This is ready to be reviewed.
Probably, but the sizes seem to be an invariant of the B-tree, so I could imagine some other places inside the B-tree do something memory unsafe because of it... at the very least, this being violated may indicate some deeper problem in the B-tree code, so it seems like an easy thing to enforce.