add Permutation_filter

[OwenConoly]

Permutation_map, Permutation_flat_map, etc. already exist, but this one seems to have been forgotten.

[andres-erbsen]

The Nix CI flocq install failure looks unrelated:

buildPhase completed in 1 minutes 21 seconds
Running phase: installPhase
Building install
mkdir: cannot create directory '/nix/store/ss49wyba13kmfx1iqqn8jab6ysgz8rd7-rocq-9.2.0/lib/coq/user-contrib/Flocq': Permission denied
Failed to build install
error: Cannot build '/nix/store/g2r6by5bz28zlvdcrbffaa3p0p49g0iq-coq9.2-flocq-dev.drv'.
       Reason: builder failed with exit code 1.
       Output paths:
         /nix/store/hisn77xzjqig6v82cgmq4w9945iw6ya9-coq9.2-flocq-dev

Merging.