std: use correct iterator to erase std modules from package.loaded.
std: use correct iterator to erase std modules from package.loaded.

* lib/specl/std.lua: With stdlib v40, std.functional.filter
returns a flat list that must be iterated over by std.list.elems
when passed through to v40 std.functional.map.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

gvvaughan/specl