Consider making the default policy for `const T&` arguments more friendly for non-copyable classes in callbacks

Follow-up from #1240 (#1200).

We disable copying and moving (not that moving is relevant here) for a set of virtual classes, because we would like to have a compile-time guarantee that we are not slicing classes. This generally has no issue in pybind, until callbacks are used, in which case the const T& instance may not be registered, and thus pybind tries to copy it, and then throws a runtime error.

Potential solutions:

• Specify callback arguments' return-value-policies (per @jagerman's suggestion) when casting a function.
• Modify the type_caster_base::cast(const T&) to check for automatic and if the class is non-copyable and non-movable, default to reference rather than copy.
  • This seems like it might be backwards-incompatible, but possibly not (as copying would be unsuccessful anyways).

My workaround for the time being will be to wrap any callbacks (Python callbacks cast into std::function<...>) that have const T& references to use const T* instead.

Read the related issues #1240 and #1200 first, then inspect type_caster_base::cast(const T&) and the callback-casting path mentioned in the issue. Decide between callback return-value policies and automatic-to-reference behavior for non-copyable types, and add regression coverage showing that a const-reference callback avoids the runtime copy error without breaking existing behavior.