Atomic transcript output file rewrites

[ChrisPenner]

Overview

@dolio recently had an "idemptotent" transcript file get wiped out because of a ctrl-c at a non-opportune moment.
This makes output-writing atomic on unix systems, and it should be slightly less likely to cause trouble on windows as well.

Implementation notes

• Add helper to the prelude for atomically replacing a file, which writes to a temp file then does a rename over the destination
• Uses this new helper in the transcript output writer

Test coverage

Existing transcripts should serve as proof this is working.

I also tried making some transcript changes to ensure they took effect.

[ChrisPenner]

Ugh, yeah looks like Windows problems

[dolio]

What if instead of a temp file, you temporarily created the output.md, then renamed it over the original .md? Not as general purpose, but might work for this case.