Add invmod algorithm

[unseddd]

Add invmod algorithm using Extended Euclidean Algorithm

Based on implementation from wolfSSL (fp_invmod directly below fp_invmod_slow)

[cuviper]

I'm sorry, but I can't accept code that was translated from GPL-licensed code:
https://www.gnu.org/licenses/gpl-faq.html#TranslateCode

There's a generic implementation in rust-num/num-integer#10, and assuming we move that to a trait (on Integer or something new) then we can optimize it further here for num-bigint. (But that still must be clean from incompatibly-licensed code!)

[unseddd]

I'm sorry, but I can't accept code that was translated from GPL-licensed code

No worries, I wasn't aware this library is not compatible with GPL. I was using similar code in another project, so figured I would try porting it upstream.

Just trying to help out.

There's a generic implementation in rust-num/num-integer#10

Cool! Hope it gets implemented for Big(U)Int, really helpful for crypto stuff.