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 = u"Comment 1"
key1 = u"the value"
key2 = 2
key3 = 5.7
key4 = True
key5 = False
key6 = 0o377
__comment2 = u"Comment no 2"
key7 = u"Umlaute: ÄÖÜäöüß"