Rework of is and as that adds new functionalities or simplify implementation

* added concepts for simplify and utilize concept subsumption rules,
* added `type_find_if` to iterate over `variant` types to simplify is/as for variant,
* rewrite `is` and `as` overloads to use `concepts` instead of `type_traits` (to utilize concepts subsumption rules),
* rewrite `narrowing`, `castable` as concepts,
* `castable` return true for types that could be directly casted with `C{x}`,
* remove `operator_is`, and `operator_as`, implementation of `is`/`as` for variants now use `type_find_if`,
* fix to ommit `to_string(...)` overload in `x as std::string`,
* fix incidental loss-casting of predicate argument e.g. `double` to `int` - now there is a check if there is a valid casting,
  * e.g. `3.14 is (:(i:int) -> _ = return i == 3;)` - after this change returns false (currently returns true),
  * the same fix for optionals: `std::optional(3.14) is (:(v:std::optional<int>)->bool = v* == 3;)`
* predicate can return type that fulfills `std::convertible_to<bool>`,
* add possibility to check if object fulfills concept by using `x is (:<T:std::integral>() = {})` syntax,
* add possibility to use free function (also generic) as predicates,
  * in case of checking against `variant` generic function will get `variant` not a type that it contains,

Author: filipsajdak