Auto merge of #512 - emilio:bitfields, r=fitzgen

Rework how bitfields are handled.

This fixes #111, and unblocks stylo.

The problem with this as of right now is that it drops the accessors (though before that this code was buggy so I'm not sure it's a loss).

I can probably try to re-implement those (though it'd be more complex). WDYT @fitzgen?

Also, note that I changed the max_align_nonce because it was incorrect (we shouldn't generate padding, because `long double` was `128` bits).