Function simplify 

pub fn simplify(arena: &mut ExprArena, id: ExprId) -> ExprId

Apply local algebraic simplifications to the subtree rooted at id, returning a (possibly fresh) ExprId that is observationally equivalent.

Current rules: constant folding for unary nodes and Pow. The linear fast-path is already canonical, so we leave Linear and n-ary Add/Mul alone.

TODO: Extend this once we add a CSE pass.