Bit-field support

[yallop]

Struct stubs should support bit-fields. Bit-fields can share the field interface, but shouldn't support the @. or |-> operators which take the address of a field.

[XVilka]

Any updates on this one? I opened an issue in ocaml-cstruct, and sent a PR, but it seems too complex to be accepted in the cstruct.

[yallop]

There hasn't been any concrete progress, but I think it's become clearer how to approach this since the issue was opened.

The challenging aspect of supporting bitfields is that the allocation order is implementation-defined, so accessing bit-fields typically needs help from the compiler. This is fairly straightforward to do with stub generation, but would be a lot more work to do using the dynamic interface. There's a little design work to do first in any case, but I think that focusing only on stub generation is likely to work out better than attempting something more dynamic or more general.