Measure and reduce the number of allocations made by Primer::Classify

[BlakeWilliams]

When investigating a page's performance related to allocations, I found that Primer::Classify was performing a significant number of allocations.

The places making the most allocations were:

• Primer::Classify#extract_class
• Hash#except
• Line 187 of Primer::Classify#extract_class

We should try to measure, then reduce the number of allocations made in that class since it can be a hot path for primer components.

[manuelpuyol]

@BlakeWilliams with #197 and #166

should we close this issue?

[BlakeWilliams]

I think we're probably good to close this for now, but can probably write some follow-up issues to prevent regressions, catch new keys, refactor, etc.

Thanks for the reminder!