Add proc composition feature (CRuby-2.6 compatible)

- Proc#<< and Proc#>>
  - Method#<< and Method#>>

mrbgems/mruby-method/mrblib/method.rb

@@ -17,4 +17,12 @@ class Method
   def name
     @name
   end
+
+  def <<(other)
+    ->(*args, &block) { call(other.call(*args, &block)) }
+  end
+
+  def >>(other)
+    ->(*args, &block) { other.call(call(*args, &block)) }
+  end
 end

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

@@ -39,4 +39,12 @@ class Proc
     make_curry.call
   end
 
+  def <<(other)
+    ->(*args, &block) { call(other.call(*args, &block)) }
+  end
+
+  def >>(other)
+    ->(*args, &block) { other.call(call(*args, &block)) }
+  end
+
 end