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.
Yep, you've got it. However we'd need to carry around some extra state to remember if this is a "real" root-to-leaf path, or a "fake" one created by this method, and then have some conditional behaviour to decide whether to do a "normal" remove call to Vec, or whether to do it manually using unsafe code. I'm not sure that the extra complexity is worth it, especially since ~50% of your elements are in leaves anyway.