A combination of an NpcKind (representing an outer species to generate), and
a function that generates a fresh Body of a species that is part of that
NpcKind each time it’s called. The reason things are done this way is that
when parsing spawn strings, we’d like to be able to randomize features that
haven’t already been specified; for instance, if no species is specified we
should randomize species, while if a species is specified we can still
randomize other attributes like gender or clothing.