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/legalizer/mod.rs

@@ -73,7 +73,7 @@ pub fn legalize_function(func: &mut ir::Function,
             match isa.encode(&pos.func.dfg,
                              &pos.func.dfg[inst],
                              pos.func.dfg.ctrl_typevar(inst)) {
-                Ok(encoding) => *pos.func.encodings.ensure(inst) = encoding,
+                Ok(encoding) => pos.func.encodings[inst] = encoding,
                 Err(action) => {
                     // We should transform the instruction into legal equivalents.
                     let changed = action(inst, pos.func, cfg);