Use custom comparison function instead of heap-lang's `=` in membership functions.

2 jobs for master in 9 minutes and 54 seconds
Status Job ID Name Coverage
  Build
failed #35812
fp-timing
build-coq.8.9.0

00:01:28

failed #35811
fp
build-coq.dev

00:09:54

 
Name Stage Failure
failed
build-coq.dev Build
Makefile.coq:320: recipe for target 'all' failed
make[1]: *** [all] Error 2
Makefile:6: recipe for target 'all' failed
make: *** [all] Error 2

real 0m28.514s
user 0m53.596s
sys 0m2.612s
ERROR: Job failed: exit code 1
failed
build-coq.8.9.0 Build
Makefile.coq:326: recipe for target 'all' failed
make[1]: *** [all] Error 2
Makefile:6: recipe for target 'all' failed
make: *** [all] Error 2

real 0m29.628s
user 0m56.432s
sys 0m2.288s
ERROR: Job failed: exit code 1