Add a constant `tc_opaque` for type class opaque definitions.

4 jobs for master
Status Job ID Name Coverage
  Build
canceled #3723
fp-timing
build-coq8.6

canceled #3722
fp-timing
build-coq8.6.1

canceled #3721
fp-timing
build-coq8.7

 
  Opam
canceled #3724
opam