Add multi-threading / multi-processing

[adamfarquhar]

The 0.5.x implementation appears to be IO bound. Throughput would be increased by moving file-reads to a separate thread so that they will happen in parallel with pattern matching.

One approach: add multiple workers, each of which reads, matches. Another: add a pool to do reads, and another to do matches.

But - it's all fast enough for now!

[carlwilson]

Delayed until API implemented, delayed until 1.7+

[carlwilson]

Hackathon 2023 review. Not selected for initial prioritisation. It could be best done when other issues are fixed, as it may be a trickier merge. Worth considering if this isn't just as well done via an invocation script