Redesign output handling to use file descriptor redirection.

[dimo414]

• Improves fidelity; nul bytes are now supported
• Saves memory; output is no longer read into a temporary variable
• Simplifies invalidation; descriptors remain valid even if files are cleaned up

[dimo414]

@zbentley mind taking a look at this? Curious if you have any suggestions or feedback (or if this is even sane...).

[dimo414]

Thanks for taking a look! This fell of my radar for a bit, but I'll plan to merge it shortly.