Memoize call_stack registry for same key entry

[vprigent]

With this simple trick, a local rails server with less than a dozen SQL queries using AR drastically reduces the amount of memory allocated by bullet.

before:

allocated memory by gem
-----------------------------------
    796102  activesupport-7.2.1
    726428  bullet/lib

after:

allocated memory by gem
-----------------------------------
    794350  activesupport-7.2.1
    313180  sprockets-4.2.1
    259688  activerecord-7.2.1
    204936  bullet/lib

Using the patched perf/benchmark.rb file to work with sqlite instead of mysql, I saw reproductible numbers:

main

Querying & Iterating 1000 Posts with 10000 Comments and 100 Users                            
user     system      total        real
14.503545   0.638605  15.142150 ( 15.201747)

patch

Querying & Iterating 1000 Posts with 10000 Comments and 100 Users

user     system      total        real
12.781415   0.664749  13.446164 ( 13.533855)