Add spec for PRFxN and SHAKE

[karthikbhargavan]

This PR adds specs to the PRFxN and SHAKE APIs in hash_functions.rs.

Note: SHAKE APIs are WIP
Note: hax throws an error at this line:

libcrux/libcrux-ml-kem/src/hash_functions.rs

Line 451 in 847e138

// #[ensures(|out|

Fatal error: exception ("Map.of_alist_exn: duplicate key" ((name K) (id (Cnst 0))))
Raised at Base__Error.raise in file "src/error.ml" (inlined), line 9, characters 14-30
Called from Base__Map.Tree0.Of_foldable.of_foldable_exn in file "src/map.ml", line 1451, characters 8-113
Called from Base__Map.Using_comparator.of_alist_exn in file "src/map.ml", line 2800, characters 25-63
Called from Base__Map.of_alist_exn in file "src/map.ml" (inlined), line 2954, characters 23-84

[github-actions]

This PR has been marked as stale due to a lack of activity for 60 days. If you believe this pull request is still relevant, please provide an update or comment to keep it open. Otherwise, it will be closed in 7 days.