Add is_binary guard to String.length (#6430)
Add is_binary guard to String.length (#6430)

This produces a bit more readable message if not a string is passed into `String.length`, then failing in the `next_grapheme_size` function.
kelvinst/elixir,lexmag/elixir,kimshrier/elixir,pedrosnk/elixir,antipax/elixir,kimshrier/elixir,beedub/elixir,ggcampinho/elixir,lexmag/elixir,michalmuskala/elixir,pedrosnk/elixir,antipax/elixir,gfvcastro/elixir,joshprice/elixir,gfvcastro/elixir,beedub/elixir,ggcampinho/elixir,kelvinst/elixir,elixir-lang/elixir