Makefile: fix dependencies
(important)
omd{,_representation,_backend,_utils,_lexer,_parser}.ml: improving the code (mostly make more things work with fsplit, which has been enhanced and moved to omd_utils.ml).
(Hopefully proper) Implementation of paragraphs in list items (which means there is now Ulp and Olp in addition to Ul and Ol in Omd.element).
tests/cow/*: fix