C++ simple implementation of RESTART_CASE like functionality from CommonLisp

Common Lisp exception handling is more advanced than in C++, it allows not only catch and handle exceptions in top-level code, but to take some decisions at lower-level and to continue work of lower-level code from the point of exception.

Imagine situation when at low level a json parser stucks at incorrect number (there is a letter between digits, for instance). It cannot decide, important this number for high-level code or not. If it will throw an exception, whole parsing process will be cancelled. But with restart-case functionality, high-level code might decide to ignore error, replace wrong-parsed number by default value and continue parsing.

Of couse, there are a lot of ways to implement such functionality in C++, but this one raher simple and inspired by the most advanced error-handling system among programming languages (Common List restart-case).

TODO

Implement subtyping, i.e. when function can specify signature of handler it accepts, without using templates:

struct case1{};
struct case2{};

using restart_case_sig1 = restart_case_signature<
    case1, std::function<int(double, char*)>,
    case2, std::function<double(short, int>
>;

// default restart_case_signature just raises exceptions

int my_func(data* some_data, const restart_case_sig& rc = restart_case_sig{})
{
...
}

Implement an algebra over restart_cases:
```
rc3 = rc1 + rc2
rc2 = rc1 - std::tuple<case1, case2>
rc2 = rc1 * std::tuple<case1, case2>
```
it is needed for specialisation of handlers through call stack, when mor abstract top-level handlers are extended and passed down to lowe-level functions.

vasil-sd/cpp_restart_case

C++ simple implementation of RESTART_CASE like functionality from CommonLisp

TODO