IO subsystem leaves partially extracted values in value store and cache

[jonahbeckford]

For portability the ThunkIo system is stream-based. But still need atomic operations.

• Copying files should be a first-class operation (use native Win32/Linux/macOS copy file operations provided by OS)
• Add a rename operation to ThunkIo, so that all writes go to a temp file and only at close (or a new finalize_exn function) does the temp file get renamed.