Allow more parallelism than rayon default (thread per core)

[mhansen]

Ripunzip uses rayon default thread pool, which I believe is one thread per logical core.

This is appropriate for cpu-bound work. However, unzipping spends a lot of time blocking on I/O, waiting for disk or network.

In such cases, you benefit from more threads than cores, so you can issue more iops in parallel. Particularly with SSDs that require many parallel I/O requests to saturate the hardware.

Ripgrep could either:

• provide a tuning argument like -j
• or auto tune the thread pool size (somehow)
• or both

[mhansen]

Need to fix #116 first, before this is important.