Proper fix for directory creation

[nigredo-tori]

Changes:

• Rewrite directory creation for Nim 0.15.3+ to remove the race (guard the new logic with a when to preserve compatibility)
• In old directory creation branch check that the resulting path is, indeed, a directory. In 0.15.2 and older createDir succeeded if the file with the same name existed.

See:

• Relevant changes in os module: nim-lang/Nim@2f725e9
• Comment mentioning the createDir bug: RFC: More flexible directory creation nim-lang/Nim#4917 (comment)
• Issue this is supposed to fix (previously closed because of a temporary fix): mkdtemp should not return pre-existing directories #5