(feat) Added hello Ingot and example.

This gives a base example of implementing an `Ingot`, and is meant to be used as a seed to facilitate faster `Ingot` implementations.

@reem, @theptrk, @mcreinhard Can you look this over and make sure it has all of appropriate syntax and style that we would expect for our library? Naming conventions, etc.

I haven't added tests or benchmarking, those will go in in later commits.

I also purposely implemented both `enter` and `exit`, to make their existence obvious.
Change these to _req, res, _alloy?