C API: update options w/ convenience funcs & fifo compaction

made rockdb_fifo_compaction_options_t type in case options are added later, if there's no plan of that it would be nice to not have it since there's only one option. 
Simply trying to follow convention from `rocksdb_universal_compaction_options_t` since they're similar ideas. Will make change but think `rocksdb_universal_compaction_options_t` should be updated accordingly for uniformity's sake (and fewer pointers).