Improve custom holder support

[dean0x7d]

This PR resolves issues #585 and #605.

1. Adds a helper struct to abstract away the holder's .get() function as suggested by @aldanor in Working with a homebrewed shared pointer type that does not implement .get() #585. This is only required for type's with a renamed .get().

   An alternative solution would be to use just a function instead of the struct, but in that case, the user-provided overload would need to be placed either in the global namespace or in the same namespace as the custom holder (ADL rules) which could possibly conflict with existing functions.

2. Support for custom move-only holders. It seems that so far only std::unique_ptr worked properly. For move-only types init_holder_helper would create a new holder from a raw pointer. This works for std::unique_ptr because its type_caster specialization would call .release(). However, custom move-only holders were handled using type_caster_holder which does not call .release() leading to the issue reported in Custom owning pointer #605.

   This PR renames type_caster_holder to copyable_holder_caster and adds a move_only_holder_caster. The type_caster<std::unique_ptr<type, deleter>> is removed in favor of move_only_holder_caster which does the same job but accepts any move-only holder. Instead of calling .release() on the moved-out pointer, this is now handled by the move constructor in init_holder_helper. This is a more portable solution since the custom holder type might not have a .release() function.

   An alternative solution would be to handle all holders using one type_caster_holder, but this would require some ugly SFINAE to disable .load() for move-only holders and for cast() to accept copyable holders only by const holder_type & and move-only by holder_type &&.

[pschella]

I would give this a different name to make explicit that one doesn't want to do this for std::unique_ptr. E.g. CustomUniquePtr.

[dean0x7d]

Good point. I'll rename it.

[pschella]

Perhaps add a two argument version to dispatch?

[dean0x7d]

I'm not sure I follow. Two argument version of what?

[pschella]

I mean adding static void init_holder_from_existing(instance_type *inst, const holder_type *holder_ptr) to dispatch between the two options.

[dean0x7d]

I don't think another layer of abstraction is needed here since this is all implementation detail and the intent should be clear from init_holder_helper right below.

[dean0x7d]

This PR also fixes #608. The issue there was that type_caster<std::unique_ptr<type, deleter>>::cast() got return_value_policy::move but the only viable policy when casting a holder is take_ownership. The new move_only_holder_caster::cast() applies the correct policy unconditionally.

[wjakob]

This looks great, thank you!