Added InternTable

[zesterer]

I was unable to use the macro for trait impls since it does not support lifetimes.

[droundy]

This commit looks like it lacks documentation and tests. Both are needed for me to accept the pull request.

I think that I can see the goal, which is to enable arena allocated interned pointers. I think in this case you should be able to implement it without unsafe, though.

[zesterer]

Yep, that's fair. I'll try to find some time to push this to completion over the next week.

[droundy]

I've finally gotten around to taking a look at this, and it looks interesting. When I look at it, however, it seems like the borrow checker would prevent you from interning more than one object in a table, since the lifetime of the first entry would make the compiler think a borrow of the table is ongoing.

I haven't tested this, but it's worth putting in an example or test to demonstrate that it's not a problem.