add test vectors of IETF draft bls signature scheme

[perturbing]

Closes #626

This PR adds test vectors for our BLS signature implementation.

[perturbing]

This can be merged after #629

[lehins]

Aside from issue with SECP256K1_ENABLED flag this PR looks good

[lehins]

Moving this module out of this cabal flag will cause it to fail compilation if DSECP256K1_ENABLED is turned off.

I am not quite sure why it was developed behind a cabal flag, but we don't want to break it all of a sudden. I suggest we investigate whether we can remove this flag and make secp256k a permanent feature without ability to opt-out.

Although, that will have to be done in a separate PR.

[lehins]

Suggested change

	let result =
	case uncheckedAggregateVerKeysDSIGN vKeys of
	Left err -> Left err
	Right avk -> verifyDSIGN ctx avk msg sig
	let result = do
	avk <- uncheckedAggregateVerKeysDSIGN vKeys
	verifyDSIGN ctx avk msg sig

[lehins]

If we want this module to be always visible, then we need to hide all secp256k functionality behind SECP256K1_ENABLED CPP flag

If we are to preserve this flag we should really have a separate CI build with it disabled