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; -*-

# a subordinate duplicate key

%YAML 1.1
---

toplevel-key:
  subkey1: the value

  subkey2: 2nd value

  subkey1: not allowed