The default container is empty. We need a manual implementation of
Default because deriving it seems to imply that K and V generic
parameter types must also implement Default.
Cloning can be used to clone an empty container or for cloning the whole
forest. We can derive this trait because we already require Copy for K
and V.
The iter() methods return an iterator that traverses all set elements /
map key-value pairs. The iterator doesn't require a mutable container
and forest reference, unlike the cursor types.
Add new ordered set and map data structures based on B+-trees. These are
not general-purpose data structures like the BTreeSet and BTreeMap types
in the standard library. They are specialized for:
- Keys and values are small `Copy` types, optimized for 32-bit entities.
- Each set or map has a very small footprint, using only 32 bits of
memory when empty.
- Keys are compared using a borrowed comparator object which can provide
context for comparing tiny types that don't contain enough information
to implement `Ord`.
- A whole forest of B-trees can be cleared in constant time without
having to traverse the whole data structure.
