Drop core::bits and merge content into core.

[partim]

Now that there is a separate domain::core crate, it feels like the bits module is an unnecessary extra indirection and could be dropped, merging all its contents into domain::core directly.

This would require some changes in use statements all over the code using domain::core. As a mitigation, we could keep bits for a while and re-export what already is in there now.

[pusateri]

I think a flag day would be fine. Just announce when you're going to do it, make the change, and make a new release.

[partim]

It’ll probably happen in 0.5.0 together with the changes I am working on to be able to use the crate in no_std environments (the octet-slices branch). While I am trying to do that this as transparently as possible, there’s bound to be some porting involved there, too.

[partim]

Never mind the no_std thing. It feels to much of a burden for the regular use case which I imagine to be more common. So instead I just went ahead and merged bits in #32.