Fix ambiguous Region/Locset expressions (#1629)

- Add a set of test cases to check the behaviour
- Remove the function `(nil)` from the the DSL
- Remove `nil` and `()` as literals
- Add functions `(region-nil)` and `(locset-nil)`

Fixes an issue where `join` (and likely `intersect`) would not work due to the fact that
`nil` values could not be coerced to `region` or `locset`. This occurred while loading 
a NML files.

While investigating it was found that certain calls, eg `(join () ())` are ambiguous and cannot
be resolved without changes in no proportion to the gain. The cause here is that
we can resolve `()` as region or a locset and at the same time `join` maps a list of
locsets or regions to a locset or region, consequently `(join () ())` can be both.

For future reference, in `label_parse.cpp::eval` we look for the first match in the `eval_map` 
of a function name that can be successfully evaluated. However, this might not be the best match
(and the ordering depends on the internal...