Dynamically requiring implementations at runtime in this way is not safe
in a multithreaded program, even in MRI with the GIL. We can simplify
this by just using autoload and turning the @@class_map into a simple
case statement.

Fixes sparklemotion#27

Repro:

    require 'http/cookie'

    # We only care about the first exception, not all of them
    Thread.report_on_exception = false

    100.times.map do
      Thread.new do
        HTTP::CookieJar::HashStore.new
      end
    end.each(&:join)

The above will reliably trigger an exception on all versions of
TruffleRuby and many versions of JRuby.  For example:

        $ ruby -v
        truffleruby 22.3.1, like ruby 3.0.3, GraalVM CE Native [x86_64-linux]
        $ ruby repro.rb
        /home/cbrasic/repos/http-cookie/lib/http/cookie_jar/abstract_store.rb:50:in `initialize': undefined method `each_pair' for nil:NilClass (NoMethodError)
                from repro.rb:10:in `block (2 levels) in <main>'

        $ jruby --version
        jruby 9.1.2.0 (2.3.0) 2016-05-26 7357c8f OpenJDK 64-Bit Server VM 25.372-b07 on 1.8.0_372-b07 +jit [linux-x86_64]
        $ jruby repro.rb
        NameError: can't resolve constant HashStore after loading http/cookie_jar/hash_store
              const_missing at /home/cbrasic/.asdf/installs/ruby/jruby-9.1.2.0/lib/ruby/gems/shared/gems/http-cookie-1.0.5/lib/http/cookie_jar.rb:25
          block in repro.rb at repro.rb:10

gemspec excludes sqlite3 from installing under jruby, so the
mozilla_store.rb file will raise LoadError under JRuby.

To avoid an undefined constant under jruby define a fake one that just
raises.

    <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85: warning: <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85: warning: loading in progress, circular require considered harmful - /home/cbrasic/repos/http-cookie/lib/http/cookie_jar.rb
            from /home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/gems/3.3.0+0/gems/rake-13.0.6/lib/rake/rake_test_loader.rb:6:in  `<main>'
            from /home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/gems/3.3.0+0/gems/rake-13.0.6/lib/rake/rake_test_loader.rb:6:in  `select'
            from /home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/gems/3.3.0+0/gems/rake-13.0.6/lib/rake/rake_test_loader.rb:21:in  `block in <main>'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'
            from /home/cbrasic/repos/http-cookie/test/test_http_cookie.rb:2:in  `<top (required)>'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'
            from /home/cbrasic/repos/http-cookie/test/helper.rb:4:in  `<top (required)>'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'
            from /home/cbrasic/repos/http-cookie/lib/http/cookie.rb:7:in  `<top (required)>'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'
            from /home/cbrasic/repos/http-cookie/lib/http/cookie_jar.rb:325:in  `<top (required)>'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'
            from /home/cbrasic/repos/http-cookie/lib/http/cookie_jar/abstract_store.rb:115:in  `<top (required)>'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'
            from /home/cbrasic/repos/http-cookie/lib/http/cookie_jar/hash_store.rb:2:in  `<top (required)>'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'
            from <internal:/home/cbrasic/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/rubygems/core_ext/kernel_require.rb>:85:in  `require'

Per request here:
sparklemotion#43 (review)