Change Omd.fallback = string to Omd.fallback = < to_string : string ; to_t : t >

Using directly a couple of functions (instead of an object with 2 methods)
doesn't work because it breaks comparison.

	modified:   omd.mli
	modified:   omd_backend.ml
	modified:   omd_parser.ml
	modified:   omd_parser.mli
	modified:   omd_representation.ml
	modified:   omd_representation.mli