Add Modular Inverse Trait

[goldenMetteyya]

No description provided.

[cuviper]

Is this a work in progress? I would at least expect an implementation of the trait for the primitive types.

Since this is specific to integers, it probably belongs in the num-integer crate. You can see that rust-num/num-integer#10 even started an inverse function.

[cuviper]

Please take this to num-integer if you want to work further.