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/binemit/relaxation.rs

@@ -67,7 +67,7 @@ pub fn relax_branches(func: &mut Function, isa: &TargetIsa) -> Result<CodeOffset
             }
 
             while let Some(inst) = cur.next_inst() {
-                let enc = cur.func.encodings.get_or_default(inst);
+                let enc = cur.func.encodings[inst];
                 let size = encinfo.bytes(enc);
 
                 // See if this might be a branch that is out of range.