Generate upcase and downcase tables from Unicode data (bug#24603)

Use Unicode data to generate case tables instead of mostly repeating
them in lisp code.  Do that in a way which maps ‘Dz’ (and similar)
digraph to ‘dz’ when down- and ‘DZ’ when upcasing.

https://debbugs.gnu.org/cgi/bugreport.cgi?msg=89;bug=24603 lists all
changes to syntax table and case tables introduced by this commit.

* lisp/international/characters.el: Remove case-pairs defined with
explicit Lisp code and instead use Unicode character properties.

* test/src/casefiddle-tests.el (casefiddle-tests--characters,
casefiddle-tests-casing): Update test cases which are now working
as they should.