Shaman was made on Linux, using the `path` module, which only supports
forward slashes. This mostly replaces `path` with `path/filepath` to use
native paths and increase Windows compatibility.
The work isn't done yet, though.
This is a better way to check errors, both semantically (no usually nil is
bad, but nil error is good, so this avoids a mental negation) and for
the reporting by the assert package.
