Investigate storing spy outputs in memory

Spy output is currently stored on disk, but they could theoretically be stored in memory as `_shpy_${spy_name}_stdout_${index}` to avoid disk access

Try looking into the following:

- Can a proof of concept be made for this?
- Should this be hidden behind a `SHPY_RUN_MODE=memory|disk` flag, or made default?
- What impact does this have on runtime of shpy's tests?

Don't forget to update `CONTRIBUTING.md` to detail the new inner workings!