Move ParamsWithStats here

[penelopeysm]

Maybe a better name -- VNTWithStats?

This struct gets used both in the PPL as well as in FlexiChains, so it makes a lot of sense to upstream it.

[penelopeysm]

Claude suggested just Draw, and also suggested using parameters and extras to mimic FlexiChains' language, which I like, so I've done that.