refactoring class-hash to be ractor-safe

mutable constants can't be shared across ractors; this changes that design to define the required variables as constants on the Resource class, which makes them reachable using ractors; the ClassHash is kept in order not to break integrations relying on its existence, but under the hood it's doing the same thing

lib/resolv.rb

@@ -2107,7 +2107,14 @@ class Resource < Query
 
       attr_reader :ttl
 
-      ClassHash = {} # :nodoc:
+      ClassHash = Module.new do
+        module_function
+
+        def []=(type_class_value, klass)
+          type_value, class_value = type_class_value
+          Resource.const_set(:"Type#{type_value}_Class#{class_value}", klass)
+        end
+      end
 
       def encode_rdata(msg) # :nodoc:
         raise NotImplementedError.new
@@ -2145,7 +2152,9 @@ def hash # :nodoc:
       end
 
       def self.get_class(type_value, class_value) # :nodoc:
-        return ClassHash[[type_value, class_value]] ||
+        cache = :"Type#{type_value}_Class#{class_value}"
+
+        return (const_defined?(cache) && const_get(cache)) ||
                Generic.create(type_value, class_value)
       end