Improve compile error for incorrect function passed in observe, modify etc

[YarikTH]

Description

I made a mistake with passing 2 args lambda to observe a single signal.

int main()
{
    ureact::context ctx;
    
    ureact::value<int> a = make_value( ctx, 1 );
    ureact::value<int> b = make_value( ctx, 1 );

    auto obs = ureact::observe( a + b, []( int a, int b ){} );
}

I received the following error message:

In file included from D:/mingw32/lib/gcc/i686-w64-mingw32/7.3.0/include/c++/bits/move.h:54:0,
                 from D:/mingw32/lib/gcc/i686-w64-mingw32/7.3.0/include/c++/bits/nested_exception.h:40,
                 from D:/mingw32/lib/gcc/i686-w64-mingw32/7.3.0/include/c++/exception:143,
                 from D:/mingw32/lib/gcc/i686-w64-mingw32/7.3.0/include/c++/ios:39,
                 from D:/mingw32/lib/gcc/i686-w64-mingw32/7.3.0/include/c++/ostream:38,
                 from D:/mingw32/lib/gcc/i686-w64-mingw32/7.3.0/include/c++/iostream:39,
                 from D:\Workspace\ureact\tests\src\playground\main.cpp:8:
D:/mingw32/lib/gcc/i686-w64-mingw32/7.3.0/include/c++/type_traits: In substitution of 'template<class _Fn, class ... _Args> using invoke_result_t = typename std::invoke_result::type [with _Fn = main()::<lambda(int, int)>; _Args = {int}]':
D:/Workspace/ureact/include/ureact/ureact.hpp:2284:44:   required from 'ureact::v0::observer ureact::v0::detail::observe_impl(const ureact::v0::signal<R>&, in_f&&) [with in_f = main()::<lambda(int, int)>; S = int]'
D:/Workspace/ureact/include/ureact/ureact.hpp:2326:42:   required from 'ureact::v0::observer ureact::v0::observe(ureact::v0::signal<R>&&, in_f&&) [with in_f = main()::<lambda(int, int)>; S = int]'
D:\Workspace\ureact\tests\src\playground\main.cpp:22:61:   required from here
D:/mingw32/lib/gcc/i686-w64-mingw32/7.3.0/include/c++/type_traits:2836:72: error: no type named 'type' in 'struct std::invoke_result<main()::<lambda(int, int)>, int>'
     using invoke_result_t = typename invoke_result<_Fn, _Args...>::type;
                                                                        ^
In file included from D:\Workspace\ureact\tests\src\playground\main.cpp:10:0:
D:/Workspace/ureact/include/ureact/ureact.hpp: In instantiation of 'ureact::v0::observer ureact::v0::detail::observe_impl(const ureact::v0::signal<R>&, in_f&&) [with in_f = main()::<lambda(int, int)>; S = int]':
D:/Workspace/ureact/include/ureact/ureact.hpp:2326:42:   required from 'ureact::v0::observer ureact::v0::observe(ureact::v0::signal<R>&&, in_f&&) [with in_f = main()::<lambda(int, int)>; S = int]'
D:\Workspace\ureact\tests\src\playground\main.cpp:22:61:   required from here
D:/Workspace/ureact/include/ureact/ureact.hpp:2289:39: error: no type named 'type' in 'struct std::invoke_result<main()::<lambda(int, int)>, int>'
     using node_t = std::conditional_t<std::is_same_v<void, R>,
                                       ^~~

It seems that I need a proper compile-time diagnostic to blame the user for incorrect usage, and give him a hint to fix it.
std::is_invocable to the resque.