Implement a `perform_allocation_under` matcher about rspec-benchmark

[piotrmurach]

I could add an alias, but I don't want to bloat API with additional methods. If you have a minute I'd appreciate doc's changes to explain this better.

[piotrmurach]

Hi Julian,

Thanks for using this gem!

The perform_allocation matcher uses the number of allocations as the upper boundary already. So your scenario should work, if it doesn't please let me know. Exact match wouldn't be very useful, only in very limited scenarios and would potentially lead to brittle tests as different Ruby versions change internally how many objects are allocated for some operations.

I will update the documentation to explain this better.

[julianrubisch]

That’s what I suspected.

Maybe add an alias that clarifies this?