get xmr-holder to send a proof of funds before ether is locked
there is a potential for a bad actor to make a monero swap offer, but without actually having the funds. then some eth holding party may come, lock their funds, and then be forced to eventually refund.

getting the xmr-holder to send a proof of funds (https://www.getmonero.org/resources/developer-guides/wallet-rpc.html#get_reserve_proof) in their initial message, which the counterparty will then verify, prevents the case where the xmr-holder didn't have the funds to begin with. however, it doesn't prevent against the case where they don't lock the funds. it also forces the xmr-holder to reveal the address that initially holds the funds, which is not ideal.

I might not end up implementing this due to the above drawbacks, needs some thought.
