Add path utilities to resolve relative paths

This makes it easy to take a buffer containing a path with relative
references (i.e. .. or . path segments) and resolve all of those
into a clean path.  This can be applied to URLs as well as file
paths which can be useful.

As part of this, I made the drive-letter detection apply on all
platforms, not just windows.  If you give a path that looks like
"c:/..." on any platform, it seems like we might as well detect
that as a rooted path.  I suppose if you create a directory named
"x:" on another platform and want to use that as the beginning
of a relative path under the root directory of your repo, this
could cause a problem, but then it seems like you're asking for
trouble.

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