Enforce buy_token != sell_token at Element parsing level

[e00E]

Hmm, good question. The exchange currently rejects buyToken == sellToken so it would also make sense for it to be an error in the Element parsing. That being said, it would require buy and sell properties on TokenPair to be private (so an invalid token pair can't be created), and for it to have a constructor that returns an Option which is significantly more work. I'm OK with the current implementation for now, but a checked TokenPair type that only accepts valid token pairs is more "correct" IMO.

Originally posted by @nlordell in #1421 (comment)