Implement sequence tracking

[bodgit]

Based on the MIT Kerberos implementation. It tracks the 64 previously seen
sequence numbers for the purposes of out of order delivery and replay
protection.

Example usage:

var payload messages.EncAPRepPart
if err := payload.Unmarshal(b); err != nil {
        return err
}

ss := gssapi.NewSequenceState(payload.SequenceNumber, flags & gssapi.ContextFlagReplay != 0, flags & gssapi.ContextFlagSequence != 0, true)

...

var token gssapi.MICToken
if err := token.Unmarshal(b); err != nil {
        return err
}

if err := ss.Check(token.SndSeqNum); err != nil {
        return err
}

Fixes #419