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/cursor.rs

@@ -157,7 +157,7 @@ impl<'c, 'f> ir::InstInserterBase<'c> for &'c mut EncCursor<'f> {
         // Assign an encoding.
         match self.isa
                   .encode(&self.func.dfg, &self.func.dfg[inst], ctrl_typevar) {
-            Ok(e) => *self.func.encodings.ensure(inst) = e,
+            Ok(e) => self.func.encodings[inst] = e,
             Err(_) => panic!("can't encode {}", self.display_inst(inst)),
         }