Containers_codegen.BitfieldSourceMake a new bitfield with the given name.
field_bit ty name adds a field of size 1 to the bitfield ty, with name name. The generate code will provide get/set for a boolean.
field_int ty name ~width adds a field of size width to the bitfield with name name. The accessors will be for integers of width bits, and the setter might assert that the provided integer fits.