Add metadata support to ExUnit

Defining setup/teardown/test explicitly is now deprecated.
They should now be defined via their respective macros:

    def setup do
      ...
    end

Now becomes:

    setup do
      ...
    end

When defined using the new format, setup callbacks can
return metadata:

    setup do
      [do_this: :do_that]
    end

And now the test macro can access this metadata:

    test "check metadata", data do
      assert data[:do_this] == :do_that
    end