* It adds a `join_paths` function that is the converse to `split_paths`,
easing manipulation of the `PATH` environment variable according to
platform conventions.
* **Breaking change**: It changes `split_paths` to no longer drop empty paths, since they are
meaningful to some shells (where they are synonymous with the current
working directory).
* It changes `setenv` to take a `BytesContainer` rather than a `&str`
value, since environment variables may have non-utf8 values on some
platforms. Since `&str` is a `BytesContainer`, this is *not* a
breaking change.
Along the way, it also refactors the `split_paths` function so that
`cfg` switches are applied internally (and the function header is given
only once). This fixes a bug: the doc comment had an example for only
one platform.