Allow BLS verification as batch in one call

[nikkolasg]

There's a bunch of calls from Go <-> FFI <-> Rust instead of just "one" call that does all transformations and check the sig in one call.
One concrete example is the hash to curve: Go first call the rust library to hash all messages to the curve, that means, the rust code creates a G2 point for each messages, and then marshal it bytes and give it back to Go. Then the Go lib passes these bytes back to rust again, but rust needs to unmarshal it back to a point ! You loose a bunch of time here - verifying a slice of bytes is a valid point often requires an expensive check - multiexp - not sure how its done here.

I do think it would be possible to just pass it all at once to Rust and let it verify by itself. If there is no other reason not to do it.