`data_synth` infinite loop when decomposing projection function

This is causing infinite loop ``` #check (fwdFDeriv ℝ fun x : ℝ => let y := foo x; let y1 := y.1; y1) rewrite_by simp -zeta only [fwdFDeriv_simproc] ``` The issue is that `data_synth` is not cheking nontrivial decomposition for `Expr.proj` correctly.