Add a way to add multiple UTXOs to an exit about contracts

[atvanguard]

Design:

1. Use a particular Dapp predicate to start an exit. There will be a small grace period for a user to reference more inputs.
2. Within the grace period, user will make 1 tx per input UTXO.

A concrete e.g. wrt our current system,

1. Let's say I start an exit from an incoming transfer (in-flight tx) using the ERC20Predicate, claiming I received B tokens of a particular type. That'd register an exit with B tokens.
2. I might already have had C tokens of the same type on the side chain. Then I'll make another tx (within the grace period) referencing my previous balance to add C tokens to my current exit. Now I am attempting to exit with B + C tokens.

The challenger can challenge either of the inputs to the exit. See params exitId and inputId in challengeExit.

[atvanguard]

For now, exits support giving upto 2 UTXOs.