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/entity/mod.rs

@@ -5,6 +5,14 @@
 //!
 //! Various data structures based on the entity references are defined in sub-modules.
 
+mod keys;
+mod map;
+mod primary;
+
+pub use self::keys::Keys;
+pub use self::map::EntityMap;
+pub use self::primary::PrimaryMap;
+
 /// A type wrapping a small integer index should implement `EntityRef` so it can be used as the key
 /// of an `EntityMap` or `SparseMap`.
 pub trait EntityRef: Copy + Eq {