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/flowgraph.rs

@@ -25,7 +25,7 @@
 
 use ir::{Function, Inst, Ebb};
 use ir::instructions::BranchInfo;
-use entity_map::EntityMap;
+use entity::EntityMap;
 use std::mem;
 
 /// A basic block denoted by its enclosing Ebb and last instruction.