Cut string allocations when defining examples.

My changes here cut string allocations when examples
are defined by 70%.
Cut string allocations when running examples.

This cuts the string allocations by ~50%.
Add allocation benchmark for many groups w/ 1 example.

I’m not sure how to address the high number of allocations
here yet.
Merge pull request #1738 from rspec/allocation-optimization

Allocation optimization
Add changelog for #1738.

[ci skip]