Specialize code for BigInt right shift by 1
Specialize code for BigInt right shift by 1

Improves ECDSA by 2-3% due to improving the const time modular
inversion algorithm (used for the mod-order inversions).

webmaster128/botan,webmaster128/botan,randombit/botan,webmaster128/botan,randombit/botan,webmaster128/botan,webmaster128/botan,randombit/botan,randombit/botan,randombit/botan