unictype/combining-class: Rename source files.

* lib/gen-uni-tables.c (main): Emit unictype/combiningclass.h instead
of unictype/combining.h.
* lib/unictype/combiningclass.c: Renamed from lib/unictype/combining.c.
Update.
* lib/unictype/combiningclass.h: Renamed from lib/unictype/combining.h.
* modules/unictype/combining-class (Description): Fix.
(Files, Makefile.am): Update.
* tests/unictype/test-combiningclass.c: Renamed from
tests/unictype/test-combining.c.
* modules/unictype/combining-class-tests (Files, Makefile.am): Update.