Investigate storing spy outputs in memory

[codehearts]

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!