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

[mcserep]

#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.