when parallelism is disabled, don't use an intermediate buffer

[BurntSushi]

When --threads 1 is given to ripgrep, it should not write results to an intermediate buffer. In particular, if every search is serialized (as it is when --threads 1 is given), then no synchronization is needed to print to stdout, and therefore, no intermediate buffer is needed.

This should be somewhat straight-forward, since the searcher is generic over a Printer<W: Terminal>.

In #4, I ended up doing this, but only when there was a single file (or stdin) to be searched.