Discover NIFs at startup

[filmor]

This is a proof-of-concept for a mechanism that does away with the need to list all NIF functions in the rustler::init! invocation. A similar mechanism will be required and implemented for resource type registrations.

• Convert Nif to a struct
• Use inventory to register the Nif instances
• In rustler::init!, ignore the passed functions and instead use the registered Nif instances

Next steps:

• [x] See whether linkme is not a better option (creates the array directly at build-time, no need to leak) (/edit Looking at this again, don't think this adds much, the current implementation is fine)
• [x] Implement resource registration using this mechanism (#617)
• [ ] See if we can move the macros to rustler (might do this in another iteration, no need to put it in the same PR)

[scrogson]

Love this idea!

[filmor]

From my point of view, this is done and ready to merge.

[filmor]

I'll merge this now. We can further discuss on #620 and/or #617 whether and how to use linkme instead.