Split EntityMap into entity::PrimaryMap and entity::EntityMap.

The new PrimaryMap replaces the primary EntityMap and the PrimaryEntityData
marker trait which was causing some confusion. We now have a clear
division between the two types of maps:

- PrimaryMap is used to assign entity numbers to the primary data for an
  entity.
- EntityMap is a secondary mapping adding additional info.

The split also means that the secondary EntityMap can now behave as if
all keys have a default value. This means that we can get rid of the
annoying ensure() and get_or_default() methods ther were used everywhere
instead of indexing. Just use normal indexing now; non-existent keys
will return the default value.

lib/cretonne/src/loop_analysis.rs

@@ -2,7 +2,8 @@
 //! and parent in the loop tree.
 
 use dominator_tree::DominatorTree;
-use entity_map::{EntityMap, PrimaryEntityData, Keys};
+use entity::{PrimaryMap, Keys};
+use entity::EntityMap;
 use flowgraph::ControlFlowGraph;
 use ir::{Function, Ebb, Layout};
 use packed_option::PackedOption;
@@ -17,7 +18,7 @@ entity_impl!(Loop, "loop");
 /// Loops are referenced by the Loop object, and for each loop you can access its header EBB,
 /// its eventual parent in the loop tree and all the EBB belonging to the loop.
 pub struct LoopAnalysis {
-    loops: EntityMap<Loop, LoopData>,
+    loops: PrimaryMap<Loop, LoopData>,
     ebb_loop_map: EntityMap<Ebb, PackedOption<Loop>>,
 }
 
@@ -26,8 +27,6 @@ struct LoopData {
     parent: PackedOption<Loop>,
 }
 
-impl PrimaryEntityData for LoopData {}
-
 impl LoopData {
     /// Creates a `LoopData` object with the loop header and its eventual parent in the loop tree.
     pub fn new(header: Ebb, parent: Option<Loop>) -> LoopData {
@@ -44,7 +43,7 @@ impl LoopAnalysis {
     /// a function.
     pub fn new() -> LoopAnalysis {
         LoopAnalysis {
-            loops: EntityMap::new(),
+            loops: PrimaryMap::new(),
             ebb_loop_map: EntityMap::new(),
         }
     }