Support nb::init(<lambda>) as syntactic sugar for custom constructors

[oremanj]

Unlike the corresponding pybind11 feature, the lambda must return by value, not by pointer. This simplifies the plumbing quite a bit and allows us to typically take advantage of guaranteed copy elision, producing basically the same code that we would have gotten from the previously-recommended manual __init__ approach. Manual __init__ is still the most appropriate method if some action needs to be taken after calling the C++ constructor.

This PR fully supports alias/trampoline classes, including the option to pass separate factory functions for constructing the bound type vs the trampoline type, except that I didn't bother providing an equivalent of pybind11's init_alias for forcing to always construct the trampoline type (since we don't have that for C++-provided constructors either). If this PR is judged too complex, the easiest way to simplify it would be by dropping support for trampoline classes. I'm a bit reluctant to do that though, since there is no supported API for constructing a trampoline class using the manual approach (since detail::nb_inst_python_derived() is internal).