SignECDSA function returns two coordinates on elliptic curve.
Catenation of these coordinates is a 64 byte signature. If
one of these coordinates have less than 32 significant bytes, then
it should have leading zeros.
It may be misleading when verify function takes signature as a hash
parameter. This commit suggested to use rfc6979 original naming
for the parameters:
- `msg` as the message to sign,
- `sig` as the signature of message.
All hashing operations are encapsulated inside of the Sign
and Verify functions.
Also there are comment fixes and re-usage of `hashBytes()` in rfc6979.
