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/libsyntax/ext/mtwt.rs

@@ -20,7 +20,7 @@ use ast::{Ident, Mrk, Name, SyntaxContext};
 use std::cell::RefCell;
 use std::rc::Rc;
 use std::collections::HashMap;
-use std::collections::hashmap::{Occupied, Vacant};
+use std::collections::hash_map::{Occupied, Vacant};
 
 /// The SCTable contains a table of SyntaxContext_'s. It
 /// represents a flattened tree structure, to avoid having