Fix benchmark LoadError in benchmark scripts

The benchmark default gem became bundled in Ruby v4.0 [1]. It needs to be added
to the Gemfile to allow the standalone benchmark scripts to run, but it doesn't
need to be added as a dependency in the gemspec.

Note that there doesn't appear to be a simple way to run all of the benchmark
scripts and indeed some of them don't appear to run successfully even in
Ruby v3.

[1]: https://rubyreferences.github.io/rubychanges/4.0.html#default-gems-that-became-bundled

Author: floehopper

Gemfile

@@ -3,6 +3,7 @@ source 'https://rubygems.org'
 
 group :development do
   if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('4.0.0')
+    gem 'benchmark'
     gem 'irb'
     gem 'logger'
   end