correct formatting on heap_print

src/prolog/heap_print.rs

@@ -56,14 +56,13 @@ pub struct PrinterOutputter {
     contents: String
 }
 
-
 impl HeapCellValueOutputter for PrinterOutputter {    
+    type Output = String;
+
     fn new() -> Self {
         PrinterOutputter { contents: String::new() }
     }
 
-    type Output = String;
-
     fn append(&mut self, contents: &str) {
         self.contents += contents;
     }