use `open_constr` in `simpl rewrite`
`simpl rewrite` with `Arguments` solves the rewrite problems in Factorization
add comments about Set Debug Tactic Unification
Use records, not typeclasses, for subuniverses and modalities
Merge pull request #583 from mikeshulman/rewrites

'simpl rewrite' can solve the problems in Factorization
Merge pull request #585 from mikeshulman/modalrecords

Use records, not typeclasses, for subuniverses and modalities