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_test.go

@@ -1,21 +0,0 @@
-package tai64n
-
-import (
-	"testing"
-	"time"
-)
-
-/* Testing the essential property of the timestamp
- * as used by WireGuard.
- */
-func TestMonotonic(t *testing.T) {
-	old := Now()
-	for i := 0; i < 10000; i++ {
-		time.Sleep(time.Nanosecond)
-		next := Now()
-		if !next.After(old) {
-			t.Error("TAI64N, not monotonically increasing on nano-second scale")
-		}
-		old = next
-	}
-}