As an alternative to forMatchingSignatures, which calls the given
function on entities matching the given Components/Tags in the
signature, forMatchingIterable allows filtering entities by an iterable
of indices that corresponds to Components/Tags.
EC/Manager and EC/Bitset can now handle types that are not known
(Components or Tags not in the given Components or Tags when created).
Manager::getEntityData now returns a pointer instead of a reference.
Functions passed to Manager::forMatchingFunction (and other similar fns)
should now accept Component fields as pointer types, not reference
types.
