* Replace SSABuilder's variables HashMaps with EntityMaps.
Current measurements show that memory usage is approximately the same,
and it's about 20% faster.
* Add EntityMap::with_default and use it.
* rustfmt
* Use var_defs[block].expand().
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.
