Make sure we write, close, and only then exec

[jahkeup]

Issue #, if available:

Fixes #29

Description of changes:

This addresses unreliable behavior where a userdata script may be partially written but not fully written, in the intended file, to disk.

Supporting refactors (for testing) and minimal test added.

@dpakpak plans to take the torch and add some more test cases - thanks!!

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