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/librustdoc/html/render.rs

@@ -34,7 +34,7 @@
 //! both occur before the crate is rendered.
 
 use std::collections::{HashMap, HashSet};
-use std::collections::hashmap::{Occupied, Vacant};
+use std::collections::hash_map::{Occupied, Vacant};
 use std::fmt;
 use std::io::fs::PathExtensions;
 use std::io::{fs, File, BufferedWriter, MemWriter, BufferedReader};