calculates a * A + b * B B is the Ed25519 base point (x,4/5) with x positive.
a[0]+256*a[1]+...+256^31 a[31].
b[0]+256*b[1]+...+256^31 b[31].
r = a * A + b * B
See Implementation
calculates a * A + b * B B is the Ed25519 base point (x,4/5) with x positive.