Reduce memory allocations for `StrongMemoize`

What does this MR do and why?

The strong_memoize is one of the most frequently used methods. Doing a few improvements we can significantly reduce GC pressure (4x) and CPU pressure (1.66x).

Comments:

• As shown by the https://gitlab-org.gitlab.io/-/gitlab/-/jobs/2147998302/artifacts/coverage/index.html#1d4d1359e610fa76c91f0dd77b07994b9140eb42 the strong_memoize is called 40_501_201 times for specs...
• The main usage pattern of strong_memoize is to pass strong_memoize(:name), but the strong_memoize("name") is possible as well
• The change retains and tests both (string and symbol)
• Previously, each call would use 4 heap slots, since the ivar was executed 2x
• Previously, the ivar would do name.to_s and create a new string with "@#{name}", this for symbol would result in two heap slots
• Now, the implementation is optimised to have exactly single allocation
• Now, the implementation is optimised to call ivar exactly once

Benchmark

  #strong_memoize
    memory allocation
      for Symbol
        performant
                     user     system      total        real
legacy non-nil   0.742533   0.013884   0.756417 (  0.756915)
legacy nil       0.734473   0.000594   0.735067 (  0.735587)
new non-nil      0.463679   0.003887   0.467566 (  0.468097)
new nil          0.441397   0.000000   0.441397 (  0.441593)
      for String
        performant
                     user     system      total        real
legacy non-nil   0.567696   0.000000   0.567696 (  0.568327)
legacy nil       0.564160   0.000000   0.564160 (  0.564414)
new non-nil      0.447927   0.000000   0.447927 (  0.448299)
new nil          0.439982   0.000000   0.439982 (  0.440183)

benchmark.rb

[:method_name, "method_name"].each do |argument|
  context "for #{argument.class}" do
    it 'performant' do
      n = 1_000_000

      result = Benchmark.bm(14) do |x|
        x.report("legacy non-nil") do
          object.clear_memoization(argument)

          n.times do
            object.legacy_strong_memoize(argument) { 10 }
          end
        end

        x.report("legacy nil") do
          object.clear_memoization(argument)

          n.times do
            object.legacy_strong_memoize(argument) { nil }
          end
        end

        x.report("new non-nil") do
          object.clear_memoization(argument)

          n.times do
            object.strong_memoize(argument) { 10 }
          end
        end

        x.report("new nil") do
          object.clear_memoization(argument)

          n.times do
            object.strong_memoize(argument) { nil }
          end
        end
      end
    end
  end
end

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.