proposal: single template to check castability and return result as std::optional

Issue description

I find myself often checking whether a particular py::object wraps this or that c++ class, and if it does, extract the result. This can be done with explicit try-catch block, like

py::object obj;
shared_ptr<T> i;
try { i=std::cast<shared_ptr<T>>(obj); } except(...) { };
if(i) // use i;    

or yet worse, with the conversion done twice:

bool ok=true;
try { std::cast<shared_ptr<T>>(obj); } except(...) { ok=false };
if(ok){
   std::cast<shared_ptr<T>> i=(obj);
   /* .... */
}

What I propose is to have a template class encompassing this dual functionality (check whether the conversion is possible, return the result if it is). boost::python did something similar with extract::check() and extract::operator() but with c++17, it would be natural to use std::optional for this.

py::object obj;
py::optional_cast<T> i(obj);
if(i){ /* use i.value() */ };

or yet more compact

if(auto i=py::optional_cast<T>(obj)){ /* use i.value() */ }

Would this fit pybind11's design?

Review the existing py::object casting interface and the proposed optional_cast usage, including the try-catch and std::optional examples in the issue. Determine whether a single castability-and-result API fits pybind11's design and what behavior would define a complete proposal.