Turn buildCfg into a method

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

lean/Spa/Language/Graphs.lean

@@ -167,10 +167,10 @@ export GGraph (comp link loop skipto singleton wrap loop_inputs loop_outputs)
 end Graph
 
 open Graph in
-def buildCfg : Stmt → Graph
+def Stmt.cfg : Stmt → Graph
   | .basic bs => singleton [bs]
-  | .andThen s₁ s₂ => buildCfg s₁ ⤳ buildCfg s₂
-  | .ifElse _ s₁ s₂ => buildCfg s₁ ∙ buildCfg s₂
-  | .whileLoop _ s => loop (buildCfg s)
+  | .andThen s₁ s₂ => s₁.cfg ⤳  s₂.cfg
+  | .ifElse _ s₁ s₂ => s₁.cfg ∙ s₂.cfg
+  | .whileLoop _ s => loop s.cfg
 
 end Spa