Semantic action should use `operator()` instead of `operator[]`

[saki7]

It is very common to use lambds in semantic actions. Currently, the syntax requires the user to add extra parenthesis around the lambda because the token [[ is ambiguously recognized as the starting character for [[attribute.

constexpr auto p = some_rule[([](auto&& ctx) { do_something(); })];

This is very, very awkward and cumbersome. We should simply use operator() and deprecate operator[].

As a consequence, this fix will going to make directives the only components which use operator[]. So it might actually make the code more readable because people can recognize that the entity is a directive as soon as they see the symbol [ in their parser definitions.

[saki7]

Another point that needs consideration is that as_parser: x4::as<T>(p) currently uses operator(). I think it was simply a mistake and should be changed to operator[].