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; mode: yaml; indent-tabs-mode: nil; -*-
%YAML 1.1
---

key1: in the root namespace
key2: in the root namespace -- too

tree1:
  key3: 0x20
  tree2:
    key4: get this as `tree1.tree2.key4'
    key5: true
    key6: 'off'