Add Enumerator support to String#each_byte.

`String#each_byte` is not defined in ISO Ruby but it is implemented in
the core mruby because it's useful.

mrblib/string.rb

@@ -167,6 +167,7 @@ class String
   ##
   # Call the given block for each byte of +self+.
   def each_byte(&block)
+    return to_enum(:each_byte, &block) unless block
     bytes = self.bytes
     pos = 0
     while pos < bytes.size