Make accessExpression private

accessExpression has no meaning in the quoted form, so don't have it be
a public rule.
Update atom tests

Update testData due to making matchedExpression and accessExpression
private.
Quote and verify literal atoms

Instead of having to run `Code.string_to_quoted` manaully to check that
Elixir.bnf generates the same parse tree as Elixir does natively, have
PsiElements support calling `quote`, which will return a quoted form in
the same format as returned by `Code.string_to_quoted`.  This requires a
second project, `intellij_elixir` to be used which sets up a gen_server
to listen for requests from erlang's JInterface.  Instructions are in
the CONTRIBUTING.md.