Use COFEs only for the recursive domain equation solver

In particular, make sure we always try eassumption before reflexivity.

The old name didn't make much sense. Also now we can have
pure_False too.

That range includes tabs and new lines.

Thanks Morten for spotting this problem.

This way we can use set_solver to solve goals involving ∈.

This has bothered me repeatedly in proofs, now I finally got around to fix it at the source