Separate parse-time C++ metrics information into distinct tables

#689 introduced a `mccabe` field into the `CppFunction` entity type, and #714 will add a similar new field for the bumpy road complexity metric.

These metric values are not coupled tightly to a `CppFunction`, and therefore they should be refactored into a separate table. Depending on the outcome of #720, this could be a *1:1* relation or a *1:many* relation.