Get rid of DIRECTORY_SEPARATOR

This is a first tentative of standardizing on `/` as the directory separator (in Windows too...) and render the usage of DIRECTORY_SEPARATOR unneeded.

It **seems** to work but thorough testing is needed, especially under Windows.

I'm unsure of what I did with unit tests: they should be particularly screened and tested
Wouldn't suggest it.  Blunt opinion (ask around, I'm full of them :wink: ), this whole PR is overkill.  Include statements have been working just fine without replacing slashes, let's not start down this path.  Our issues have been heavily in the area of path comparisons, I'd say focus in first on areas which are doing just that and branch out.