Avoid Python U256 decoder repetition?

[thibault-martinez]

As seen in #1552, things like

    amount: int = field(metadata=config(
        encoder=hex,
        decoder=lambda v: int(v, 16)
    ))

should probably be factored.
Maybe something like

def HexField(**args):
    return field(metadata=config(
        encoder=hex,
        decoder=lambda v: int(v, 16),
        **args,
    ))

[thibault-martinez]

Closed by #1557