fix(ckb): load the first 16 bytes of the XUDT cell data

[ShookLyngs]

Changes

• Resolves Use first 16 bytes of xUDT cell data as an amount #211

Reference

ref: https://blog.cryptape.com/enhance-sudts-programmability-with-xudt#heading-xudt-cell

XUDT cell
On top of sUDT, xUDT extends a cell as follows:

data:
    <amount: uint128> <xudt data>