Move classpath to rb_classext_t

This commit moves the classpath (and tmp_classpath) from instance
variables to the rb_classext_t. This improves performance as we no
longer need to set an instance variable when assigning a classpath to
a class.

I benchmarked with the following script:

```ruby
name = :MyClass

puts(Benchmark.measure do
  10_000_000.times do |i|
    Object.const_set(name, Class.new)
    Object.send(:remove_const, name)
  end
end)
```

Before this patch:

```
  5.440119   0.025264   5.465383 (  5.467105)
```

After this patch:

```
  4.889646   0.028325   4.917971 (  4.942678)
```

1 files changed, 8 insertions, 2 deletions

diff --git a/spec/ruby/optional/capi/object_spec.rb b/spec/ruby/optional/capi/object_spec.rb
index 25a43d8908..9efc892202 100644
--- a/spec/ruby/optional/capi/object_spec.rb
+++ b/spec/ruby/optional/capi/object_spec.rb
@@ -993,13 +993,19 @@ describe "CApiObject" do
       end
 
       it "calls the callback function for each cvar and ivar on a class" do
+        exp = [:@@cvar, :foo, :@@cvar2, :bar, :@ivar, :baz]
+        exp.unshift(:__classpath__, 'CApiObjectSpecs::CVars') if RUBY_VERSION < "3.3"
+
         ary = @o.rb_ivar_foreach(CApiObjectSpecs::CVars)
-        ary.should == [:__classpath__, 'CApiObjectSpecs::CVars', :@@cvar, :foo, :@@cvar2, :bar, :@ivar, :baz]
+        ary.should == exp
       end
 
       it "calls the callback function for each cvar and ivar on a module" do
+        exp = [:@@mvar, :foo, :@@mvar2, :bar, :@ivar, :baz]
+        exp.unshift(:__classpath__, 'CApiObjectSpecs::MVars') if RUBY_VERSION < "3.3"
+
         ary = @o.rb_ivar_foreach(CApiObjectSpecs::MVars)
-        ary.should == [:__classpath__, 'CApiObjectSpecs::MVars', :@@mvar, :foo, :@@mvar2, :bar, :@ivar, :baz]
+        ary.should == exp
       end
 
     end