Defer None loading to second pass

Many of our `is_none()` checks in type caster loading return true, but
this should really be considered a deferral so that, for example, an
overload with a `py::none` argument would win over one that takes
`py::none` as a null option.

This keeps None-accepting for the `!convert` pass only for std::optional
and void casters.  (The `char` caster already deferred None; this just
extends that behaviour to other casters).