[bindings] Add support for `Option<T>` where T is a mapped trait

[bindings] Add support for `Option<T>` where T is a mapped trait

When mapping an `Option<T>` where T is a mapped trait, we need to
move out of the `*mut T`, however the current generation results in
a `*const T` and a conversion that just takes a reference to the
pointed-to object. This is because the only place this code was
previously used was for slices, which *do* need a reference.

Additionally, we need to know how to convert `Option` containers
which do not contain an opaque type.

Sadly, the easiest way to get the desired result is to add another
special case in container mapping, keeping the current behavior for
slices, but moving out of the pointed-to object for other types.