Treat complete Windows UNC prefixes as absolute paths

[dlenski]

I justify this in #61, specifically:

\\some.server\share should be considered equivalent to
\\some.server\share\, with .is_absolute() being true regardless of
whether or not the final \ is included. In the same way that
C:\whatever and C:\whatever\ are equivalent when .joining or
.pushing further path components.

[chipsenkbeil]

@dlenski good catch! Seems like there's a minor nitpick in clippy about using !is_empty() over len() > 0.

Outside of that, wondering out loud if we need to also add more tests for \\?\UNC\server\share (VerbatimUNC) and \\server (server-only), and maybe update our absolutize() test to have a case for this as well.

[dlenski]

Seems like there's a minor nitpick in clippy about using !is_empty() over len() > 0.

Fixed, and cargo fmt too.

Outside of that, wondering out loud if we need to also add more tests for \\?\UNC\server\share (VerbatimUNC) and \\server (server-only), and maybe update our absolutize() test to have a case for this as well.

Yeah. As you can see, I added a couple of relevant doctests, but I do think that more Windows-specific tests for path-joining and absolutizing would be useful, given the many strange aspects of Windows paths.