use PLNK instead of duplicating "bench" framework (dragoon/komodo!84)

i've basically refactored the whole "bench" framework that was inlined in  `examples/benches/operations/field.rs` and `examples/benches/operations/curve_group.rs` into a new repo called [PLNK](https://gitlab.isae-supaero.fr/a.stevan/plnk).

nothing effectively changes on the side of Komodo but now the code is much simpler here :)