Upper bound on how much of the account's FundingCoin the swap may spend — also the amount policy limiters are evaluated against.
Called with the Coin<FundingCoin> withdrawn from the account (a
transaction argument, not a value) — perform your swap against it
(e.g. deepbook.swapExactQuantity) and return the resulting
Coin<PlatformCoin> plus any unspent Coin<FundingCoin> change.
Composed into the same tx, between withdraw_for_route and
process_routed_payment — this is the only place a swap can go,
since the funding coin doesn't exist until withdraw_for_route runs.
The coin type the platform settles in.
The coin type the account actually holds.