Add Kernel#yield_self; CRuby2.5

mrbgems/mruby-kernel-ext/mrblib/kernel.rb

@@ -0,0 +1,13 @@
+module Kernel
+  # call-seq:
+  #   obj.yield_self {|_obj|...} -> an_object
+  #
+  # Yields <i>obj</i> and returns the result.
+  #
+  #   'my string'.yield_self {|s|s.upcase} #=> "MY STRING"
+  #
+  def yield_self(&block)
+    return to_enum :yield_self unless block
+    block.call(self)
+  end
+end