Count_occurrences doesn't correctly track regular expressions

https://github.com/QuantGov/quantgov/blob/5c22c6ee9cd1c702561b881bcf5dd38097d7ff99/quantgov/corpora/builtins.py#L123

We need the `term_counts` dictionary keys to be the actual regexes, not the string matches. We should map the one to the other and then generates results from the remapped version.