sudt's amount type should be u128

[eval-exec]

In https://github.com/nervosnetwork/rfcs/blob/master/rfcs/0025-simple-udt/0025-simple-udt.md#sudt-cell

data:
    amount: uint128
type:
    code_hash: simple_udt type script
    args: owner lock script hash (...)
lock:
    <user_defined>

sudt's amount type should be u128. Fix #105