Provide a C implementation of configmix.config._split_filters. This is needed as precondition for implementation interpolate_variables in C also. Currently the speedup is not measurable but it does not hurt also. Also provide some unit-tests for _split_filters().

{"__comment1": "Comment 1",
 "key1": "the value",
 "key2": 2,
 "key3": 5.7,
 "key4": true,
 "key5": false,
 "key6": 255,
 "__comment2": "Comment no 2",
 "key7": "Umlaute: ÄÖÜäöüß"
}