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 -*-

__all__ = ["key1", "nonexistingkey"]

key1 = u"the next value"
key2 = "non-exported value"