Cosmos tx amount is casted to uint64

[0x0ece]

Hi, while Terra and Cosmos have relatively low amount of decimals, new chains are popping up with 18 decimals, e.g. XPLA.

We just noticed that there's an explicit cast to uint64:
https://github.com/renproject/multichain/blob/master/chain/cosmos/tx.go#L120

This causes overflow issue/incorrect amount on such chains.

[jazg]

Hey @0x0ece, good catch! We should go through and update the internal amount types to pack.U256 to prevent this from happening. We will add this to the backlog, but if you would like to give it a go and create a PR that would be appreciated!