DMFF: various tweaks
DMFF: various tweaks

- Remove [strip_implicits] (doesn't make a difference)
- Fix some bug in [check_m] in the mini type-checker
- Improve the quality of some matches... but doesn't help in the specific case
  of the quality of the generated bind_wp for stexn... the issue is deeper than
  that.

FStarLang/FStar,FStarLang/FStar,hoheinzollern/FStar,YaccConstructor/FStar,hoheinzollern/FStar,hoheinzollern/FStar,A-Manning/FStar,YaccConstructor/FStar,YaccConstructor/FStar,hoheinzollern/FStar,YaccConstructor/FStar,A-Manning/FStar,A-Manning/FStar,FStarLang/FStar