Optimize String.downcase/1
Optimize String.downcase/1

Thanks to @michalmuskala for feedback and review.

michalmuskala/elixir,lexmag/elixir,pedrosnk/elixir,kelvinst/elixir,kimshrier/elixir,ggcampinho/elixir,kimshrier/elixir,kelvinst/elixir,lexmag/elixir,gfvcastro/elixir,elixir-lang/elixir,pedrosnk/elixir,gfvcastro/elixir,joshprice/elixir,ggcampinho/elixir