Implement Hash.ruby2_keywords_hash?(hash) and Hash.ruby2_keywords_hash(hash)

* It is used by pp.rb in stdlib, so we need it for nice error messages in MSpec.
* Not using it for foo(*args) calls yet.

Author: eregon

spec/tags/core/hash/ruby2_keywords_hash_tags.txt

@@ -1,6 +1 @@
-fails:Hash.ruby2_keywords_hash? returns false if the Hash is not a keywords Hash
 fails:Hash.ruby2_keywords_hash? returns true if the Hash is a keywords Hash
-fails:Hash.ruby2_keywords_hash? raises TypeError for non-Hash
-fails:Hash.ruby2_keywords_hash returns a copy of a Hash and marks the copy as a keywords Hash
-fails:Hash.ruby2_keywords_hash raises TypeError for non-Hash
-fails:Hash.ruby2_keywords_hash returns an instance of the subclass if called on an instance of a subclass of Hash

src/main/java/org/truffleruby/core/hash/HashNodes.java

@@ -173,6 +173,23 @@ public boolean isSmallArrayOfPairs(Object[] args, RubyLanguage language) {
 
     }
 
+    @CoreMethod(names = "ruby2_keywords_hash?", onSingleton = true, required = 1)
+    public abstract static class IsRuby2KeywordsHashNode extends CoreMethodArrayArgumentsNode {
+        @Specialization
+        protected boolean isRuby2KeywordsHash(RubyHash hash) {
+            return hash.ruby2_keywords;
+        }
+    }
+
+    @Primitive(name = "hash_mark_ruby2_keywords")
+    public abstract static class HashMarkRuby2KeywordsNode extends CoreMethodArrayArgumentsNode {
+        @Specialization
+        protected RubyHash markRuby2Keywords(RubyHash hash) {
+            hash.ruby2_keywords = true;
+            return hash;
+        }
+    }
+
     @ImportStatic(HashGuards.class)
     public abstract static class HashLookupOrExecuteDefaultNode extends RubyContextNode {
 

src/main/java/org/truffleruby/core/hash/RubyHash.java

@@ -29,6 +29,7 @@ public class RubyHash extends RubyDynamicObject implements ObjectGraphNode {
     public Entry firstInSequence;
     public Entry lastInSequence;
     public boolean compareByIdentity;
+    public boolean ruby2_keywords = false;
 
     public RubyHash(
             RubyClass rubyClass,

src/main/ruby/truffleruby/core/hash.rb

@@ -103,6 +103,10 @@ def self._constructor_fallback(*args)
   end
   private_class_method :_constructor_fallback
 
+  def self.ruby2_keywords_hash(hash)
+    Primitive.hash_mark_ruby2_keywords(hash.dup)
+  end
+
   alias_method :store, :[]=
 
   # Used internally to get around subclasses redefining #[]=