add bit reordering (via BitfieldConfig)

[khusmann]

I'm not sure if I like this feature, so I'm keeping it off the main branch for now and will consider adding it in later if folks convince me otherwise.

This allows you to re-order bits in a bitfield before it is parsed. So if you have a bitfield like this:

0-4: Lower 4 bits of uint8 field
4-11: Padding
12-15: Upper 4 bits of uint8 field

You can create a bitfield like this:

 class Work(bd.Bitfield):
    a: int = bd.uint_field(8)
    _pad: int = bd.uint8_field(8, default = 0)

    bitfield_config = bd.BitfieldConfig(
        reorder_bits=[*range(12, 16), *range(0, 4)]
     )

The reorder_bits option in the configuration will reorder the bits like this:

12, 13, 14, 15, 0, 1, 2, 3, (rest of bits in stream)

So that field a in the bitfield is parsed all at once.

At first glance this feature seemed nice, but in practice it got a little ugly and error-prone. In general, I think it's a better idea to just read the bitfield as is, (with an a_lower and a_upper field) and then combine them when you convert from bitfield -> domain object in your code (so use the bitfield object similar to how you'd work with an ORM object -- useful for IO, but confined to the edges of your system)