lexer: introduce a Position (Line, Column) handler

Signed-off-by: Alejandro Mery <amery@jpi.io>

lexer/lexer.go

@@ -7,16 +7,11 @@ type StateFn func() (StateFn, error)
 // Run runs a state machine until the state function either
 // returns nil or an error
 func Run(fn StateFn) error {
-	for fn != nil {
-		var err error
+	var err error
 
+	for fn != nil && err == nil {
 		fn, err = fn()
-		if err != nil {
-			// failed
-			return err
-		}
 	}
 
-	// ended
-	return nil
+	return err
 }