Added `pub` visibility mark to `mod_inverse()`
fn to be able to benchmark it.

All tests are passing and Kalinski Inverse
has been introduced on the code replacing
the last mod_inv implementation.

Bitshift to the left instead of multiply by 2
since performs ffaster than mul.

Implemented Kalinski Modular inverse algorithm.
See: https://www.researchgate.net/publication/3387259_Improved_Montgomery_modular_inverse_algorithm

Also implemented test for big and small Bignums.

One test is giving results over the Montgomery Domain, so
a little bit of research has to be done.