Refactor Resources

[filmor]

• Move storage of "Type->NIF Resource Type Handle" to a OnceLock map to allow implementing resource types without resorting to dynamic trait implementations
• Implement resource types as a derive macro
• Add direct access methods to get an immutable reference to the wrapped objects
• Add corresponding converters

This allows us to eventually deprecate the existing resource! macro to fix #606.

TODO (or follow-up):

• Incorporate [wip] add support for monitors #376 (functionality and tests added, different implementation, see Refactor Resources #617 (comment))
• Add ATO Resources #84

[filmor]

Another option, instead of a derive macro (or additional to one) would be to allow explicitly implementing a Resource trait and registering it using a rustler::resource!() outside of load, which would use the inventory::submit! under the hood. This would allow us to make the down callback for #376 (and dyncall and other potential callbacks) a trait function with an empty default implementation.

If we like the derive style, another alternative would be, to have a derive(Resource) and a derive(MonitorResource), where MonitorResource requires an implementation of a MonitorResource trait with a down_callback function and these would be different code paths with two sets of registrations. I think I'm in favour of that.

/edit: This is how it is implemented now: #[derive(MonitorResource)] + implementing the MonitorResource trait.

[filmor]

Note to self: #84 is also a very interesting approach (completely doing away with the necessary pre-registration for resource types!), I'm just not quite sure how to incorporate monitors into it.