coq 8.20 has a small regression with lia.
+ some small stdlib changes.

Dropped <=v8.11 because:
1) #[export] not supported but needed
2) Some definitions we use (e.g. Bool.le) do not exist in before
   v8.12.

Both problems are fixable but it is probably not worth the effort.

+ new typeclasses Space and Location to avoid bad resolution
+ no center field in similarities as it can be defined instead
+ homothecy fixed to leave their center invariant and not map it to the origin
+ RobotInfo.app renamed to lift

This reverts commit 1d351274b92c5c734e7a3afa528d8a724cb04aa6.