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().

# -*- coding: utf-8; -*-

__comment1 = "Comment 1"
key1 = "the value"
key2 = 2
key3 = 5.7
key4 = true
key5 = false
key6 = 0o377
__comment2 = "Comment no 2"
key7 = "Umlaute: ÄÖÜäöüß"