Eagerly defrost chilled strings

Ref: https://bugs.ruby-lang.org/issues/20205

One minor backward compatibility issue with chilled strings
is that since they claim to be frozen, code using the pattern:

```ruby
StringIO.new("")
```

Is now producing read only `StringIO` instances. It would be
preferable to emit a warning instead.

We can detect whether the current Ruby produce chilled strings
and if so preemptively attempt to mutate the given string to
produce the warning immediately and preserve the original
behavior.

Author: byroot

lib/stringio.rb

@@ -0,0 +1,19 @@
+require "stringio.so"
+
+if "test".frozen? && !"test".equal?("test") # Ruby 3.4+ chilled strings
+  class StringIO
+    alias_method :_initialize, :initialize
+    private :_initialize
+
+    def initialize(*args, **kwargs)
+      string = args.first
+      if string && string.frozen?
+        begin
+          string << "" # Eagerly defrost the string
+        rescue FrozenError
+        end
+      end
+      _initialize(*args, **kwargs)
+    end
+  end
+end

test/stringio/test_stringio.rb

@@ -981,4 +981,12 @@ def test_coderange_after_overwrite
   def assert_string(content, encoding, str, mesg = nil)
     assert_equal([content, encoding], [str, str.encoding], mesg)
   end
+
+  if "test".frozen? && !"test".equal?("test") # Ruby 3.4+ chilled strings
+    def test_chilled_string
+      io = StringIO.new("")
+      io << "test"
+      assert_equal("test", s.string)
+    end
+  end
 end