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
as you obviously can understand this has been a rough search/replace job done globally, with a cursory look at not breaking things, but without taking into account the source (our own vs. 3rd party) of the affected code.