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.
I guess this raises a point of inefficiency: removing an internal node does two in-tree byte copies of the key/value when only one is actually required; theoretically we could just directly move it out here, and leave a hole in the leaf of the free. (This is quite unsafe though; there would need to be guaranteed no-failure and the `remove_stack` call would need to be careful to `mem::forget` the return value.)