Don't use modules

Feel free to revert this if you have a strong feeling about it. But so
far as I can see, it adds a lot of complexity for basically no upsides.

internal/tai64n/tai64n.go

@@ -1,26 +0,0 @@
-package tai64n
-
-import (
-	"bytes"
-	"encoding/binary"
-	"time"
-)
-
-const TimestampSize = 12
-const base = uint64(4611686018427387914)
-
-type Timestamp [TimestampSize]byte
-
-func Now() Timestamp {
-	var tai64n Timestamp
-	now := time.Now()
-	secs := base + uint64(now.Unix())
-	nano := uint32(now.UnixNano())
-	binary.BigEndian.PutUint64(tai64n[:], secs)
-	binary.BigEndian.PutUint32(tai64n[8:], nano)
-	return tai64n
-}
-
-func (t1 Timestamp) After(t2 Timestamp) bool {
-	return bytes.Compare(t1[:], t2[:]) > 0
-}