Creatures are going to have an inventory too now in addition to
characters, so all the data types and lenses and stuff that define
inventory need to be broken out into a separate module so the Creature
entity can use them.
Change-Id: I83f1c70d316afaaf2e75901f9dc28f79fd2cd31f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3901
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
