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/regalloc/spilling.rs

@@ -453,7 +453,7 @@ impl<'a> Context<'a> {
             .make_spill_slot(self.cur.func.dfg.value_type(value));
         for &v in self.virtregs.congruence_class(&value) {
             self.liveness.spill(v);
-            *self.cur.func.locations.ensure(v) = ValueLoc::Stack(ss);
+            self.cur.func.locations[v] = ValueLoc::Stack(ss);
         }
     }