Binding constructors via factory functions

[oremanj]

The porting guide from pybind11 explains why nanobind doesn't have an equivalent of .def(py::init(<lambda>)):

Unfortunately, the implementation of this feature was quite complex and often required further internal calls to the move or copy constructor.

However, nanobind has something pybind11 didn't have: the ability to depend on C++17 features, including guaranteed copy elision. You can construct a T using a lambda-that-returns-a-T without doing any copies or moves at all, even if T isn't copyable or movable.

I'm wondering whether there might be appetite for accepting a PR that allows nb::class_<T>(...).def(nb::init(<lambda>)), only in those cases where <lambda> returns something that can direct-initialize a T. This would be intended as purely syntactic sugar for the placement-new formula, and would be expected to compile to an equivalent level of efficiency in most cases.