add support for framing

[khusmann]

So far, this is just a sketch how I might implement framing for bitfields.

Example usage:

kissFraming = SimpleFraming(
    delimiter=0xC0,
    escape_byte=0xDB,
    escape_map={
        0xC0: 0xDC,  # Frame delimiter
        0xDB: 0xDD,  # Escape byte
    },
)

class Foo(bd.Bitfield):
    a: int = bd.uint_field(4)
    b: int = bd.uint_field(4)

framer = BitfieldFramer(Foo, kissFraming)

foo = Foo(a=1, b=2)

framer.to_bytes([foo, foo, foo])
# b"\xC0\x12\xC0\xC0\x12\xC0\xC0\x12\xC0"