Add back WnafBase::multiscalar_mul#23
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Computes a sum-of-products `aA + bB + ...` in variable time with w-NAF
multi-exponentiation using the interleaved window method, also known
as Straus' method.
The key insight is that when computing this sum by means of additions
and doublings, the doublings can be shared by performing the additions
within an inner loop.
The API and implementation are inspired in part by `curve25519-dalek`,
namely the `VartimeMultiscalarMul` trait and corresponding
implementation in `straus.rs`.
This results in ~28% speedup on `p256` for a 3 scalar/point input:
ProjectivePoint operations/point-scalar lincomb (variable-time)
time: [149.13 µs 149.80 µs 150.84 µs]
change: [−27.999% −27.645% −27.267%] (p = 0.00 < 0.05)
This is closer to the `VartimeMultiscalarMul` trait in `curve25519-dalek`.
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Second take on #14