De-duplicate methods in core::path(2) when default trait methods work

[graydon]

The new path code has a bunch of duplicate code between the posix and windows variants. It should be factored into material in the abstract Path trait, when default methods work.

[lkuper]

Blocked on #2794.

[graydon]

Still legit, 2013-06-12.

(Note: #2794 may well be fixed shortly, worth revisiting soon!)

[emberian]

Default methods work now.

[lkuper]

#8909 closes this.