Optimize rstrip
Optimize rstrip

This new implementation is no longer linear without affecting
smaller samples. For a string that is 100 bytes long, it is
25x faster than the previous implementation.

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