Modernize fork hooks to not conflict with other gems

Fix: zk-ruby#108

`Module#prepend` is much more composable than alias method chains.

This is essentially a port of the Active Support implementation.

lib/zk/install_fork_hooks.rb

@@ -1,37 +1,51 @@
-module ::Kernel
-  def fork_with_zk_hooks(&block)
-    if block
-      new_block = proc do
-        ::ZK::ForkHook.fire_after_child_hooks!
-        block.call
+module ZK
+  module ForkHook
+    module ModernCoreExt
+      def _fork
+        ::ZK::ForkHook.fire_prepare_hooks!
+        pid = super
+        if pid == 0
+          ::ZK::ForkHook.fire_after_child_hooks!
+        else
+          ::ZK::ForkHook.fire_after_parent_hooks!
+        end
+        pid
       end
+    end
 
-      ::ZK::ForkHook.fire_prepare_hooks!
-      fork_without_zk_hooks(&new_block).tap do
-        ::ZK::ForkHook.fire_after_parent_hooks!
-      end
-    else
-      ::ZK::ForkHook.fire_prepare_hooks!
-      if pid = fork_without_zk_hooks
-        ::ZK::ForkHook.fire_after_parent_hooks!
-        # we're in the parent
-        return pid
-      else
-        # we're in the child
-        ::ZK::ForkHook.fire_after_child_hooks!
-        return nil
+    module CoreExt
+      def fork(*, **)
+        ::ZK::ForkHook.fire_prepare_hooks!
+        if block_given?
+          pid = super do
+            ::ZK::ForkHook.fire_after_child_hooks!
+            yield
+          end
+          ::ZK::ForkHook.fire_after_parent_hooks!
+        else
+          if pid = super
+            ZK::ForkHook.fire_after_parent_hooks!
+          else
+            ::ZK::ForkHook.fire_after_child_hooks!
+          end
+        end
+
+        pid
       end
     end
-  end
 
-  if defined?(fork_without_zk_hooks)
-    remove_method :fork
-    alias fork fork_without_zk_hooks
-    remove_method :fork_without_zk_hooks
+    module CoreExtPrivate
+      include CoreExt
+      private :fork
+    end
   end
-
-  alias fork_without_zk_hooks fork
-  alias fork fork_with_zk_hooks
-  module_function :fork
 end
 
+if Process.respond_to?(:_fork) # Ruby 3.1+
+  ::Process.singleton_class.prepend(ZK::ForkHook::ModernCoreExt)
+elsif Process.respond_to?(:fork)
+  ::Object.prepend(ZK::ForkHook::CoreExtPrivate) if RUBY_VERSION < "3.0"
+  ::Kernel.prepend(ZK::ForkHook::CoreExtPrivate)
+  ::Kernel.singleton_class.prepend(ZK::ForkHook::CoreExt)
+  ::Process.singleton_class.prepend(ZK::ForkHook::CoreExt)
+end