Merge pull request #4443 from shuujii/move-Integral-zero-nonzero-positive-negative-to-Numeric

Move `Integral#(zero|nonzero|positive|negative)?` to `Numeric`

mrbgems/mruby-numeric-ext/mrblib/numeric_ext.rb

-module Integral
+class Numeric
   def zero?
     self == 0
   end