SEC-19 ECDSA recovery id (V) is casted from uint64 to single byte

[Gustav-Simonsson]

V can be treated as an integer everywhere it is used in the Go code, which avoids possibility of truncation of high bytes when casting from uint64 to (a single) byte.

See YP Appendix F.

[obscuren]

An integer in go is not a single byte. It's either int32 or int64 depending on the host. V should always be a single byte. I'll change the implementation to take a byte rather than an uint64.