Also cleaned up implementation; only generates syntax for requested part(s) of
source location, and is more intelligent about calculation of relative paths.
Paths are left as paths, instead of trying to convert them to strings
or byte strings. Submodule path elements should be unquoted -- in the
same form as a `submod' form. All extra parts are submodule path elements,
never module paths or ".".
`setup/private/path-utils'.
The API is a little different: instead of getting the alist and the
path, there's a curried function that gets the alist and produces a
function to do the substitutions.
