mirage-crypto-ec: add Brainpool curves with 254/384/512 bits This change adds the Brainpool curves, namely brainpoolP254r1, brainpoolP384r1, brainpoolP512r1. The implementation internally uses the twisted versions of the curves (...t1) in order to use the same a=-3 EC arithmetic as the NIST curves. Point coordinates in regular form are transformed to twisted form before calculations, and vice versa afterwards.