functional: new `case` module function.

* specs/functional_spec.yaml: Add specifications for a `case`
function.
* lib/std/functional.lua (case): New function.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
package: new path management apis.

* specs/package_spec.yaml: Specify behaviour of new apis.
* lib/std/io.lua (pathsep): Recalculate rather than introduce a
require loop with std.package.
* lib/std/package.lua (find, insert, mappath, normalize, remove):
New functions for maintaining clean pathstrings.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>