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
include, include_once, require, and require_once haven't been touched.
In other cases we can "factor in" the mod in the called function/method...

I want to stress this: this is not meant **for production**, but just to test if "the thing" works in all aspects: then there will be time for "fine tuning".

Be blunt, don't worry: I always accept constructive criticism! :smile: