When using bytes.Buffer to construct network protocol buffers (not to be confused with
google protobuf), it would be useful to have functions for writing and reading integer
values, like

    - ReadInt/64
    - WriteInt/64

thus avoiding the bitshift boilerplate

    var i int64 = 0x1234567890abcdef
    var b = []byte{
        byte(i >> 56 & 0xFF),
        byte(i >> 48 & 0xFF),
        byte(i >> 40 & 0xFF),
        byte(i >> 32 & 0xFF),
        byte(i >> 24 & 0xFF),
        byte(i >> 16 & 0xFF),
        byte(i >> 8 & 0xFF),
        byte(i & 0xFF),
    }