Update `Path` constructor to new conventions

[brson]

Should have a static new method.

Subset of #3853

[brson]

More or less blocked on #4202

[thomaslee]

@brson I did some investigatory digging into this issue with my fix for #4202 in place & unfortunately I suspect that reexports of traits are still broken enough in certain cases that Path::new won't work. That said, I do plan to dig deeper into the underlying issues with trait reexports once my girlfriend forgives me for the past ~week or so of neglect. :P

[thomaslee]

@brson to be crystal clear: we're essentially removing WindowsPath & PosixPath methods, aliasing GenericPath as Path with a "pub use ..." & renaming GenericPath::from_str to GenericPath::new here, right?

[metajack]

revisiting for triage. seems correctly nominated and still an issue.