Use std filesystem for ROOT7

It looks like an std filesystem implementation is impending, so an obvious improvement for ROOT7 is to remove the current multi-platform abstractions for file access. Users can instead use the std implementations, and the existing interfaces can be redirected to use std:: during a transition period.

Less generic code in ROOT == more better! less maintenance! easier to transfer for non-ROOT C++!

Hi Jean-Fran├žois,

thanks for your remark.
Relying on what the standard library and the language provide now and will provide in the future is one of the cornerstones of the ROOT7 related activities.
You should feel invited to join the ROOT7 meetings: you find more information and the instructions about how to subscribe to the mailing list here
root.cern.ch/root-7

Cheers,
Danilo