Move Symbol#name into symbol.rb

This allows to declare it as leaf just like `Symbol#to_s`.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

string.c

@@ -12740,7 +12740,6 @@ Init_String(void)
     rb_define_method(rb_cSymbol, "==", sym_equal, 1);
     rb_define_method(rb_cSymbol, "===", sym_equal, 1);
     rb_define_method(rb_cSymbol, "inspect", sym_inspect, 0);
-    rb_define_method(rb_cSymbol, "name", rb_sym2str, 0); /* in symbol.c */
     rb_define_method(rb_cSymbol, "to_proc", rb_sym_to_proc, 0); /* in proc.c */
     rb_define_method(rb_cSymbol, "succ", sym_succ, 0);
     rb_define_method(rb_cSymbol, "next", sym_succ, 0);

symbol.rb

@@ -14,6 +14,20 @@ class Symbol
 
   alias id2name to_s
 
+  # call-seq:
+  #   name -> string
+  #
+  # Returns a frozen string representation of +self+ (not including the leading colon):
+  #
+  #   :foo.name         # => "foo"
+  #   :foo.name.frozen? # => true
+  #
+  # Related: Symbol#to_s, Symbol#inspect.
+  def name
+    Primitive.attr! :leaf
+    Primitive.cexpr! 'rb_sym2str(self)'
+  end
+
   # call-seq:
   #   to_sym -> self
   #