Implement Access.at/1 (#4719)
Implement Access.at/1 (#4719)

It was described, but not implemented, on #4009
elixir-lang/elixir,kelvinst/elixir,lexmag/elixir,kelvinst/elixir,pedrosnk/elixir,gfvcastro/elixir,kimshrier/elixir,beedub/elixir,gfvcastro/elixir,michalmuskala/elixir,ggcampinho/elixir,lexmag/elixir,ggcampinho/elixir,antipax/elixir,beedub/elixir,pedrosnk/elixir,joshprice/elixir,kimshrier/elixir,antipax/elixir