Clean up String.slice/2/3 impls
Clean up String.slice/2/3 impls

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