Fix signs.ml tutorial eval to handle negated constants via UnOp (#2014)

* Initial plan

* Add UnOp Neg case to eval in signs.ml tutorial with TODO

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* Specialize UnOp Neg case to Const in signs.ml eval

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* Apply suggestion from @michael-schwarz

* Update src/analyses/tutorials/signs.ml

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---------

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people

src/analyses/tutorials/signs.ml

@@ -60,6 +60,7 @@ struct
   (* This should now evaluate expressions. *)
   let eval (d: D.t) (exp: exp): SL.t = match exp with
     | Const (CInt (i, _, _)) -> SL.top () (* TODO: Fix me! *)
+    | UnOp (Neg, Const (CInt (i, _, _)), _) -> SL.top () (* TODO: Fix me! *)
     | Lval (Var x, NoOffset) -> D.find x d
     | _ -> SL.top ()