add: const witness

[dzmitry-lahoda]

problem

rust has very limited typing option for const parameters

solution

use witness types with const function https://github.com/ergoplatform/bounded-vec/blob/39207d4a7bfcd3f22e542bdbd7b47f650a555417/src/bounded_vec.rs#L40

to type intricate constraints mixing consts and types

[dzmitry-lahoda]

i can PR it if it is fine to review for merge