Move the responsibility of safe loading to Node

This change moves the responsibility of safely loading a YAML tree from
an internal detail of `Psych.safe_load` into `Psych::Nodes::Node`.

The advantage of this is that you can now more easily safely load trees
that you're working on without replicating the safety constraints of
`Psych.safe_load`. The delegation of responsibilities also now matches
for safely and unsafely loading a tree.

lib/psych.rb

@@ -319,20 +319,10 @@ def self.unsafe_load yaml, filename: nil, fallback: false, symbolize_names: fals
   #   Psych.safe_load("---\n foo: bar")                         # => {"foo"=>"bar"}
   #   Psych.safe_load("---\n foo: bar", symbolize_names: true)  # => {:foo=>"bar"}
   #
-  def self.safe_load yaml, permitted_classes: [], permitted_symbols: [], aliases: false, filename: nil, fallback: nil, symbolize_names: false, freeze: false, strict_integer: false
+  def self.safe_load yaml, filename: nil, fallback: nil, **kwargs
     result = parse(yaml, filename: filename)
     return fallback unless result
-
-    class_loader = ClassLoader::Restricted.new(permitted_classes.map(&:to_s),
-                                               permitted_symbols.map(&:to_s))
-    scanner      = ScalarScanner.new class_loader, strict_integer: strict_integer
-    visitor = if aliases
-                Visitors::ToRuby.new scanner, class_loader, symbolize_names: symbolize_names, freeze: freeze
-              else
-                Visitors::NoAliasRuby.new scanner, class_loader, symbolize_names: symbolize_names, freeze: freeze
-              end
-    result = visitor.accept result
-    result
+    result.to_safe_ruby(**kwargs)
   end
 
   ###

lib/psych/nodes/node.rb

@@ -51,6 +51,23 @@ def to_ruby(symbolize_names: false, freeze: false, strict_integer: false)
       end
       alias :transform :to_ruby
 
+      ###
+      # Safely convert this node to Ruby.
+      #
+      # See also Psych.safe_load
+      #
+      def to_safe_ruby(permitted_classes: [], permitted_symbols: [], aliases: false, symbolize_names: false, freeze: false, strict_integer: false)
+        class_loader = ClassLoader::Restricted.new(
+          permitted_classes.map(&:to_s),
+          permitted_symbols.map(&:to_s)
+        )
+        scanner = ScalarScanner.new(class_loader, strict_integer: strict_integer)
+        visitor_class = aliases ? Visitors::ToRuby : Visitors::NoAliasRuby
+
+        visitor_class.new(scanner, class_loader, symbolize_names: symbolize_names, freeze: freeze).accept(self)
+      end
+      alias :safe_transform :to_safe_ruby
+
       ###
       # Convert this node to YAML.
       #