rollup merge of rust-lang#22208: aturon/expose-more-path

This commit exposes the `is_sep` function and `MAIN_SEP` constant, as
well as Windows path prefixes. The path prefix enum is safely exposed on
all platforms, but it only yielded as a component for Windows.

Exposing the prefix enum as part of prefix components involved changing
the type from `OsStr` to the `Prefix` enum, which is a:

[breaking-change]

Author: alexcrichton