Try harder to show that a RAN is rational (#8230)

Try harder to show that a RAN is rational (#8230)

We try to rewrite real algebraic numbers to rationals. This is important to, e.g., allow the linear solver to see rational constants at all. Refining real algebraic numbers is done lazily, and (almost) any operation may trigger a refinement that makes libpoly realize it actually is rational. Thus, even the mere act of creating a node out of a real algebraic number may do this (it hashes it).
This PR thus introduces a fixed-point loop into the RAN node constructor that defends against this case.
Fixes #8226.