Use export-hook in core

[travisbrown]

I'm creating this issue to follow up on a comment by @dwijnand on gitter:

The only thought I had about #29 is the need for the imports, and I that made me think if maybe core should use export-hook

I've played around with export-hook in core, but I really want to push on the experiment of keeping generic derivation modular for as long as possible. I don't love my current approach of getting priorities right by balancing a tower of instance-providing traits and using a Secondary wrapper, but it seems to work reasonably well, and is still (I think) not completely unintelligible.

[travisbrown]

I'm removing the question tag since @milessabin has just convinced me that export-hook can do what I want. I'll tackle this next week unless someone beats me to it.

[travisbrown]

Fixed by #43.

[dwijnand]

👍