Generate random volumes and densities for items based on the ranges for
those two quantities in the raw when building instances of items.
Since this is the first time creating an item is impure, this also lifts
entity generation into a (random) monadic context
Change-Id: I2de4880e8144f7ff9e1304eb32806ed1d7affa18
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3226
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI
d8bd8e7eea
