lexer: introduce a Position (Line, Column) handler

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

lexer/lexer.go

@@ -7,11 +7,16 @@ 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 {
-	var err error
+	for fn != nil {
+		var err error
 
-	for fn != nil && err == nil {
 		fn, err = fn()
+		if err != nil {
+			// failed
+			return err
+		}
 	}
 
-	return err
+	// ended
+	return nil
 }