Forests of B+ trees.

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.

lib/cretonne/src/lib.rs

@@ -15,6 +15,7 @@ pub mod dbg;
 #[macro_use]
 pub mod entity;
 
+pub mod bforest;
 pub mod binemit;
 pub mod bitset;
 pub mod cursor;