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.
Good catch. This method makes a lot of assumptions about its input. In particular, it is supposed to only called on a Vec with len == cap. It should honestly just be `unsafe`. I also need to put a fail! case in BTreeMap's constructor that validates that B>1, or else the tree makes basically no sense.