crypto/ecdsa: use variable time ScalarBaseMult/ScalarMult in Verify

[rolandshoemaker]

We're leaving some perf on the table by doing these operations in constant time. We should use a variable time scalar mult, similar to what we do for crypto/ed25519:

go/src/crypto/internal/fips140/ed25519/ed25519.go

Line 335 in 1a93e4a

R := (&edwards25519.Point{}).VarTimeDoubleScalarBaseMult(k, minusA, S)

cc @FiloSottile

[gabyhelp]

Related Code Changes

• crypto/elliptic: update P-521 docs to say it's constant-time

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