All domain Registry Traversal per Backend

[DoeringChristian]

The PR adds the option to traverse all registry entries for a backend.

This is required for function freezing, as it is not easily possible to get the domains of a variable representing a pointer, when traversing a C++ class such as a scene.
The fix is to just traverse the whole registry.

This PR makes the following changes:

• modifies the jitc_registry_id_bound function to return the bound over all domains in the backend if the domain argument is a nullptr
• Adds a jitc_registry_fill_ptrs function, taking a pointer to a memory region to fill with the registered pointers.
• Adds the backend as a field to the Domain struct

[DoeringChristian]

@wjakob, @merlinND This PR should be ready for review.

[merlinND]

Thanks for making this one easy to review as well!

[wjakob]

Looks good!