Add/Improve set unfold instances.

This MR does three things:

• It adds new set unfold instances for lists and fin_map's dom and map_img.
  • A caveat is that the ones for map_img + fmap/omap are too generic to be declared as Instances, so are just lemmas.
• It improves existing instances for gmultiset's dom and set_bind.
• It fixes a render error in README on a link to docs/sets.v.

I grouped them into one MR since they are small and tied to set unfold (minus the typo, which is trivial). I can split it up if necessary.