Add helper function to populate CDict
Add helper function to populate CDict

We're going to reuse the CDict in other operations shortly. This will
reduce code reuse.
indygreg/python-zstandard,terrelln/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard