Raise when Access syntax used on non-keywords

[lexmag]

Closes #4071.

[lexmag]

Not sure if we really want to "inline" this call since we already use Keyword.get_and_update below and I don't think Access calls are meant to be used with big lists (mostly small options, params, etc.) and will be called intensively.

[josevalim]

Should we raise a nice error message like we did in 1.0 that said the
access protocol is only available to keywords? Much clearer than a function
clause error.

José Valimwww.plataformatec.com.br
http://www.plataformatec.com.br/Founder and Director of R&D

[lexmag]

I think we should make it nicer because of the call transformation [:key] –> Access.fetch what could be unclear for newcomers, and there is another case which we should consider:

iex> [{:foo, 1, 1}][:foo]
** (CaseClauseError) no case clause matching: {:foo, 1, 1}
    (elixir) lib/access.ex:57: Access.fetch/2

[lexmag]

And it nicely works together with the 1. from #4070.

[josevalim]

Yes, let's improve all those cases. #4070 will be at compile time.