Define `alias_antonym` to quickly create "opposite" methods.

`alias_antonym` allows you to define "antonym" or "opposite" methods,
which simply return the negated value of what's returned by another
method. For instance:

    class ParityChecker
      class << self
        def even?(n)
          n % 2 == 0
        end

        alias_antonym :odd?, :even?
      end
    end

   ParityChecker.even?(5) # => false
   ParityChecker.odd?(5) # => true