refactor: use atomicwrites for writing replaced file

[acheronfail]

Waiting on:

• Overwriting file on Windows gives PermissionDenied untitaker/rust-atomicwrites#35

[acheronfail]

Investigate https://docs.rs/tempfile/3.1.0/tempfile/struct.TempPath.html#method.persist as a possible alternative

[acheronfail]

After some attempts I've narrowed down the issue to this: Stebalien/tempfile#131

After running some manual tests, the atomic replacement functions correctly on Windows, just not between two instances of NamedTempFile.

[acheronfail]

Okay, this has been fixed with some workarounds:

• Need to use NamedTempFile::keep() (in tests) to let Windows then be able to write to that file
• Need to drop the File object returned from the above before attempting to atomically replace (NamedTempFile::persist()) onto that file