Fix compatibility with rustc pre-1.46

    error: unexpected token: `0.0`
       --> src/runtime.rs:186:18
        |
    186 |             self.0.0
        |             -----^^^
        |             |    |
        |             |    unexpected token
        |             help: try parenthesizing the first index: `(self.0).0`

src/runtime.rs

@@ -183,7 +183,7 @@ mod get_span {
     impl GetSpan<Span> {
         #[inline]
         pub fn __into_span(self) -> Span {
-            self.0.0
+            (self.0).0
         }
     }