Personally, I'd say that only exported symbols are usable in future cells, and
then I'd load each cell in its own module and have each module automatically
import all previous modules/cells.
Advantages:
* Very easy.
* Works without any parsing or macros.
* You get cell/module local variables as well, which could be useful.
* You're pointing to the real symbol, not a copy/recreation, so you can
update it across multiple cells and it will do the right thing automatically.
You could potentially use Nim devel and `import {.all.}` if you really want to
access unexported symbols using the same approach. I'm not positive this would
work, but it seems like it should.
