Tracking Issue: Migration to filepath 1.5

@bgamari recently sent out the email about bumping libraries before the cut of 9.10, and @Bodigrim mentioned that filepath 1.5 should probably be included:

Bump filepath submodule to 1.5 and add os-string to boot libraries. Julian might remember better, but AFAIR there are no blockers, just someone has to upgrade several submodules at once.

I was now confronted with the issue of filepath-1.5 compatibility at various places, but there was no tracking issue so far.

cc @maerwald