Proof step and variable lemmas

1. no writes = save value
2. embedding commutation with steps
3. all variables in code end up in the set of vars

lean/Spa.lean

@@ -11,6 +11,7 @@ import Spa.Language.Equivalence
 import Spa.Language.Graphs
 import Spa.Language.Traces
 import Spa.Language.Properties
+import Spa.Language.TraceProperties
 import Spa.Language
 import Spa.Analysis.Forward.Lattices
 import Spa.Analysis.Forward.Evaluation