Add convenience constexpr functions for working with preset_t as an index.
Add convenience constexpr functions for working with preset_t as an index.

pieis2pi/Slic3r,curieos/Slic3r,alexrj/Slic3r,curieos/Slic3r,pieis2pi/Slic3r,alexrj/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r,curieos/Slic3r,curieos/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r,curieos/Slic3r,curieos/Slic3r,alexrj/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r,pieis2pi/Slic3r