btcec signature Verification should pass the preimage

[dabura667]

https://twitter.com/pwuille/status/1063582706288586752?s=19

Notice: ECDSA signatures where the message isn't a hash and chosen by the "signer" are insecure.

Given public key P, pick random nonzero values a and b. Compute R=aG+bP. Now (R.x, R.x/b) is a valid signature for "message" (R.x*a/b).

Verify should take the preimage as []byte and double SHA256 it.

Current method can be renamed VerifyUnsafe.

[Roasbeef]

and double SHA256 it.

It's a generic secp256k1 library, also one can use any hash function to verify an ECDSA signature.

[dabura667]

pass a hash.Hash interface as well and perform the hash within Verify.