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