Support for a PurePath class

[mwgamble]

It would be nice to see a PurePath class, like the Python pathlib package has.

[LandonTClipp]

I think the reason I didn't implement this is that Go doesn't really have inheritance. It's tricky to make Path contain an "Is A" relationship.

type PurePath struct{}

type Path {
    PurePath
}

but what do you do in cases like .Parent()? Well, PurePath could do:

func (p PurePath) Parent() PurePath {}

but then Path.Parent is going to return a PurePath, not a Path. How do you reconcile this? I'm not sure.