We have both from and to here, so technically we can either drop the neg/neg
trick from the processTokenTransfer() or drop one field from the structure
(the other side is a part of the key). Drop the field since this can make the
DB a bit more compact. Change Amount to be a pointer along the way since
that's the "native" thing for big.Int, we've used non-pointer field
specifically to avoid Neg/Neg problems, but it looks like this is not
necessary.
This structure is only used by the RPC server and I doubt anyone uses it via
the *Blockchain.
