Implement `Cursor` API for BTreeMap

[declanvk]

See rust-lang/libs-team#141 for design of API and links to implementing CR

---

This feature may be a bit difficult because I believe that the BTreeMap implementation maintains a linked-list among leaf nodes so that traversing among those nodes is easy.

Options:

1. We also start maintaining a leaf node linked list, this would likely speed up and simplify iterators, but at the cost of increased memory usage in the tree. Currently our iterators maintain a stack of previous nodes, we could save some memory usage here
2. We could implement the cursor API using the stack based traversal method, but moving to the next node would be slightly slower.e