ENH: allow specifying bit fields in a dtype

[mattip]

ctypes allows structs to specify bit size and unions:

from ctypes import *
class myPacketHeader(Structure):
    _pack_ = 1
    _fields_ = [("seqnum", c_ubyte, 8),
                ("address", c_uint, 31),
                ("is_command", c_uint, 1),
                ("length", c_ubyte, 8)]
class myUnion(Union):
    _pack_ = 1
    _fields_ = [("header", myPacketHeader),
                ("bytes", c_uint8 * 6)]

It might be nice if we could allow this in dtypes for conversion to/from structured arrays too.

Something to think about for the future.