Currently we often use constructs like

	_, err := e.w.Write([]byte{opNone})
	return err

however with added Encoder.emit helper it can become only

	return e.emit(opNone)

which is more clearly readable.

We can do similarly for formatted output:

-	_, err := fmt.Fprintf(e.w, "%c%dL\n", opLong, b)
-	return err
+	return e.emitf("%c%dL\n", opLong, b)

Due to much boilerplate in one place (encodeInt) the return of
fmt.Fprintf was not even checked. We change the code there with

-		_, err = e.w.Write([]byte{opInt})
-		if err != nil {
-			return err
-		}
-		fmt.Fprintf(e.w, "%d\n", i)
+		return e.emitf("%c%d\n", opInt, i)

which is now hopefully more visible for what is going on and is easier
to catch potential mistake by eyes.

A corresponding test for "int64 encoded as string" will be added later.