[curves] new 25519 measurements of formally verified implementations

[Jason A. Donenfeld]

I've loaded in fiat64 into the latest kbench curve testing branch, and
it seems to be the fastest generic C version, at least on my Skylake
laptop, inching out slightly in front of hacl64:

donna64: 121790 cycles per call
hacl64: 109782 cycles per call
fiat64: 108984 cycles per call
sandy2x: 102996 cycles per call
 amd64: 108563 cycles per call
fiat32: 232826 cycles per call
donna32: 412092 cycles per call