Use `distance` for exchange

[earthiverse]

Changes:

• Clean up exchange code a bit
• Use distance instead of simple_distance for distance check

[earthiverse]

Tested locally

• Exchanging seashells near Xyn (fails correctly)
• Exchanging gem0 near Xyn (success)
• Exchanging gem0 far from Xyn (fails correctly)
• Exchanging non-exchangable (fails correctly)

[earthiverse]

About the change from simple_distance to distance, was it only for consistency or did you notice some issues?

I noticed it when I was getting range errors when exchanging.

I think I remember Wiz saying he would standardize all distance checks to use distance.