cpp-common/bt2c: add bt2c::call()

cpp-common/bt2c: add bt2c::call()

This new function partially implements INVOKE.

It's bt2c::call() instead of bt2s::invoke() because it's not a perfect
drop-in replacement (not forwarding `func` because
`std::reference_wrapper` requires that the parameter isn't an rvalue).

The std::ref() trick is taken from [1].

My main use case for this is IIFE (initializing some `const` variable
from a parameter-less lambda call) without risking to forget the
trailing `()` to actually call it:

    const auto meow = [&aqua, &fina] {
        // some steps
        return mix;
    }; // oops, forgot `()` here

Instead:

    const auto meow = bt2c::call([&aqua, &fina] {
        // some steps
        return mix;
    });

When you see bt2c::call(), you know there's a call.

You can also use bt2c::call() for most use cases [2] of std::invoke()
(C++17), for example:

    template <typename MethT, typename ObjT>
    void f(MethT&& meth, ObjT&& obj, const int val)
    {
        bt2c::call(std::forward<MethT>(meth), std::forward<ObjT>(obj), val, 42);
    }

    struct Meow
    {
        void mix(const int a, const int b)
        {
            std::cout << a << ":" << b << std::endl;
        }
    };

    int main()
    {
        Meow meow;

        f(&Meow::mix, meow, 17);
    }

[1]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0312r1.html
[2]: https://devblogs.microsoft.com/oldnewthing/20220401-00/?p=106426

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ib7db0315ee6f68f168793488a7da67c1bc410fb8
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11425
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>