Some mechanism is needed for yielding expectation failure in semantic action

[saki7]

Currently, the only way to yield expectation failure is from normal parsers:

constexpr auto p = a > b;
constexpr auto p = a >> x4::expect[b];

We need some facility to yield expectation failure in semantic action:

// proposed syntax
constexpr auto p = a >> b[([](auto&& ctx) {
    return x4::unexpected("error message"); 
})];

The current workaround is to just throw C++ exception:

constexpr auto p = a >> b[([](auto&& ctx) {
    throw std::runtime_error("error message"); 
})];

However this workaround has serious downsides:

1. It defeats the purpose of expectation failure and essentially degrades your codebase into X3 era, where it suffered from C++ exception's overheads
2. It yields "unreachable code" warning on compilers because all of the X4's internal code which comes after the x4::action handling is correctly analyzed as "unreachable code" on very well optimized compilers