Speedup ec.recoverPubKey

[fanatid]

Similar to cryptocoinjs/secp256k1-node#101
should be ~2x faster

[coveralls]

Coverage increased (+0.007%) to 88.101% when pulling ec437d0 on fanatid:speedup-ec-recoverPubKey into ab93810 on indutny:master.

[indutny]

Benchmarks? :)

[fanatid]

var EC = require('elliptic').ec
var secp256k1 = new EC('secp256k1')

var R = secp256k1.genKeyPair().getPublic()
var e = secp256k1.genKeyPair().getPrivate()
var s = secp256k1.genKeyPair().getPrivate()
var rinv = secp256k1.genKeyPair().getPrivate()

var cnt = 5e2

console.time('1 point mul')
for (var i = 0; i < cnt; ++i) {
  secp256k1.g.mulAdd(e, R, s).mul(rinv)
}
console.timeEnd('1 point mul')

console.time('2 scalar mul')
for (var i = 0; i < cnt; ++i) {
  secp256k1.g.mulAdd(e.mul(rinv).umod(secp256k1.n), R, s.mul(rinv).umod(secp256k1.n))
}
console.timeEnd('2 scalar mul')

1 point mul: 3057.023ms
2 scalar mul: 1855.858ms

[indutny]

Ok, I'm convinced. Thank you!

[indutny]

Landed in f3edfe5, and released in 6.3.2. Thank you!