Frozen string literals [experimental]

Created by: soulcutter

Attempting to resolve whether or not Ruby 3+ frozen string literals represent a performance regression in 2.3. Related to #2264

My manual testing with time rspec shows the original code took around 10s and the frozen literal code took around 9.75 seconds (it was fairly consistent over multiple runs). Hardly scientific, but good enough for me.

Requires rspec/rspec-support to also use the frozen-string-literals branch to run.