Allow marking parameters as constants? Or behave differently if they are random?

[alexandrebouchard]

E.g. take Dirichlet before commit 2509312366a2a99514c6a47092d3d0c9a9a35556, it was fine if concentration param is via a ConstantSupplier (I think!), but not if e.g. it's via Dirichlet(#[aVar, anotherVar]).

Basic solution would be to add keyword say and check it's indeed a constant supplier. Annoying part is that then implementer has to pick between efficiency and generality. E.g. BPS will definitely want a constant structure. Maybe that's OK. Maybe can detect and move loops inside. But seems hard.