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 -*-

key1 = "in the root namespace"
key2 = "in the root namespace -- too"

[tree1]
key3 = 0x20

[tree1.tree2]
key4 = "get this as `tree1.tree2.key4'"
key5 = true
key6 = "off"