Implement core::ops::Shr for BigInt in place of BigInt::divn()

[mmaker]

https://github.com/arkworks-rs/algebra/blob/master/ff/src/biginteger/mod.rs#L434
The BigInt::divn function should instead be done with overloading using the trait Shr from the standard library.
(Cf. #713)

[mmaker]

@hdvanegasm want to give it a shot? :)

[hdvanegasm]

It would be nice to implement not only core::ops::Shr but also core::ops::Shl. If I implement core::ops::Shl for BigInt<N> by moving it some bits and there is an overflow of M limbs, should I return a BigInt<N + M> or should I just return a BigInt<N> and chop the overflow?

[mmaker]

This issue is just for Shr :)