X4 should provide builtin semantic actions

[saki7]

X4 should provide these frequently used semantic actions as builtin constexpr CPO functors:

• assign to x4::get<ID>(ctx)
  • assign to _local_var
  • assign to _rule_var
  • assign to _as_var
  • assign to _attr
• class member pointer overload of above
• tuple index overload of above

The functionalities themselves are quite easy to implement. What it matters the most is the naming. We should name these objects really carefully or the users would get very confused.

For example, x4::assign_to<0> can either mean assign the 0th element of the attribute to the exposed attribute or assign the attribute to the 0th element of the exposed attribute. This kind of confusion frequently occurs in my ad-hoc grammar, so the official version should be designed very carefully.

Also, we need at least some synopsis described in our README.