Add Fermat based inversion of P-384 field elements
Add Fermat based inversion of P-384 field elements

Cuts about 100K cycles from the inversion, improving ECDSA sign by 10%
and ECDH by ~2%

Addition chain from https://briansmith.org/ecc-inversion-addition-chains-01

GH #1479

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