maint: Add xml.equal function

* dietncl/xmlsugar.lua (xml.equal): New function; checks whether two XML
trees are equal.
* test-xmlsugar-equal.lua: New file; checks xml.equal.
tests: Add util.parsenclformat function

* tests/util.lua (parsenclformat): New function; parses a formatted NCL
string.
tests: Make tests stricter

* tests/test-xmlsugar-clone.lua: Likewise.
maint: Add 'duplicated id' error message

* dietncl/errmsg.lua (messages): Likewise.
tests: Add test-xmlsugar-equal.lua to Makefile

* Makefile (TESTS): Likewise.
maint: Resolve <importNCL>; make filter.import tests stricter

* dietncl/filter/import.lua: Resolve <importNCL> elements.  Fix bugs in
the resolution of <importBase>.
* tests/test-filter-import.lua: Add checks for <importNCL>; make
tests stricter.
doc: Update to-do

* TODO: Likewise.
doc: Add README

* README: New file.