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.
Instead of a simple type synonym, this could be a struct paired with an `&'a mut BTreeMap<K, V>`. This would be safer, as it would ensure that the stack is used with the correct tree and that the stack doesn't live too long. Additionally, `insert`, `leafify`, and `remove` could be implemented as methods of `SearchStack`, making them slightly nicer to use.