refactor libcollections as part of collection reform

* Moves multi-collection files into their own directory, and splits them into seperate files
* Changes exports so that each collection has its own module
* Adds underscores to public modules and filenames to match standard naming conventions

(that is, treemap::{TreeMap, TreeSet} => tree_map::TreeMap, tree_set::TreeSet)

* Renames PriorityQueue to BinaryHeap
* Renames SmallIntMap to VecMap
* Miscellanious fallout fixes

[breaking-change]

src/test/run-fail/hashmap-capacity-overflow.rs

@@ -10,7 +10,7 @@
 
 // error-pattern:capacity overflow
 
-use std::collections::hashmap::HashMap;
+use std::collections::hash_map::HashMap;
 use std::uint;
 use std::mem::size_of;