Add signature marshaling/unmarshaling functions

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I have to do

r, s = rfc6979.SignECDSA(key, h[:], sha256.New)
var sig = make([]byte, 2*32)
r.FillBytes(sig[0 : 32])
s.FillBytes(sig[32:])

when using this package.

Describe the solution you'd like

Add function to marshal/unmarshal the sig.