split completion from COFEs and only require it for the solver

This is the OFE part of !22 

Cc @robbertkrebbers @jjourdan

See merge request !26

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.