This is the opposite of unwrap(). It converts the ad-hoc null references like
NO_EBB and NO_INST into the more standard Option<Ebb> type which unfortunately
takes twice as much space in data structures.
This supports the pattern of creating structs wrapping a u32 and using them as
indexes into a vector of entities. These entity references should implement the
EntityRef trait.
The EntityMap is a generic map from an EntityRef to some value type. It expects
densely indexed entities and uses a Vec to represent the mapping compactly.