Resolve used_underscore_items pedantic clippy lint

    warning: used underscore-prefixed item
       --> src/backtrace.rs:410:5
        |
    410 |     _assert::<Backtrace>();
        |     ^^^^^^^^^^^^^^^^^^^^^^
        |
    note: item is defined here
       --> src/backtrace.rs:409:5
        |
    409 |     fn _assert<T: Send + Sync>() {}
        |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#used_underscore_items
        = note: `-W clippy::used-underscore-items` implied by `-W clippy::pedantic`
        = help: to override `-W clippy::pedantic` add `#[allow(clippy::used_underscore_items)]`

src/backtrace.rs

@@ -406,6 +406,6 @@ mod capture {
 }
 
 fn _assert_send_sync() {
-    fn _assert<T: Send + Sync>() {}
-    _assert::<Backtrace>();
+    fn assert<T: Send + Sync>() {}
+    assert::<Backtrace>();
 }