extend results on ceil 'for free'

[fdupress]

This opaquely defines ceil from floor so all results on ceil apply to floor and don't need copied. We still start copying them.

This is minimally done for the purpose of a specific proof for now; feel free to ask me to make it draft for now.

[strub]

Overall, this is ok. But I don't understand why this PR breaks external devs.

[fdupress]

I don't either, I'll draft it until I understand what I need for the dev anyway.

I expect some brittle SMT solvers are made cranky by the fact that there is slightly more transparency here. (And if some solvers or their why3 driver defines floor in terms of ceil, for example, we could have some looping that evades detection during saturation.)

[strub]

I on't remember wether opaque definitions are sent to SMT. I'll check.