Take a String by move into error_string()

src/lexer/mod.rs

@@ -98,7 +98,7 @@ impl Lexer {
         self.input[self.begin .. self.forward].to_string()
     }
 
-    fn error_string(&self, message: &str) -> String {
+    fn error_string(&self, message: String) -> String {
         format!("{}:{}: {}", self.line, self.line_offset, message)
     }
 }