Add `crosspath.ToNative(path)`, which returns the path with platform-
native path separators. This is meant for use in the Worker, to convert
paths before attempting to use them.
Both Go's standard `path` and `path/filepath` packages are too limiting to
work well for Flamenco. The former assumes Linux/POSIX paths, the latter
only works with platform-native paths. Neither can work with Windows paths
on Linux, or Linux paths on Windows.
