Add then alias to yield_self.

`then' was added in CRuby 2.6.

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

@@ -1,6 +1,7 @@
 module Kernel
   # call-seq:
   #   obj.yield_self {|_obj|...} -> an_object
+  #   obj.then {|_obj|...}       -> an_object
   #
   # Yields <i>obj</i> and returns the result.
   #
@@ -10,4 +11,5 @@ module Kernel
     return to_enum :yield_self unless block
     block.call(self)
   end
+  alias then yield_self
 end