maint: Comment fixes

* dietncl/filter/import.lua: Likewise.
maint: Add util.uequal function

* tests/util.lua (util.uequal): New function; similar to xml.equal, but
ignores the order of elements.
maint: Add filter.region module

* dietncl/filter/region.lua (filter.region): New filter; remove region
elements.
* tests/test-filter-region.lua: New test; checks filter.region.
* Makefile (TESTS): Updated accordingly.