Refactor the DIY equational reasoning library to be a bit more like
the generic pre-order reasoning library:
Change the `...` notation into a constructor for a new `Step` datatype.
This seems to help idris disambiguate between the two kinds of
reasoning when they're used in the same file (e.g., frex).
Co-authored-by: Ohad Kammar <ohad.kammar@ed.ac.uk>
For lack of a better place, I've put it in `Syntax.PreorderReasoning`
These equations are natural in equational reasoning, but less so when
rewriting, so that's why it's there
