Fixed endianness in Curve25519 implementation (no consequence on security). Also added new Curve25519 code for 64-bit platforms.
Cosmetic fix (value did not conform to its announced bit length, but this did not have bad consequences since br_i31_decode_mod() is lenient on that).
Added optimised implementation of P-256 that uses 32->64 multiplications (MUL31).
Two new Curve25519 implementations (generic "i31" code, and optimised code with MUL31).