Bypass anyhow_kind-based dispatch for ensure! with default msg

[dtolnay]

For ensure!(1 < 2), Before:

if !(1 < 2) {
    return $crate::private::Err({
        use $crate::private::kind::*;
        match "Condition failed: `1 < 2`" {
            error => (&error).anyhow_kind().new(error),
        }
    });
}

After:

if !(1 < 2) {
    return $crate::private::Err($crate::private::new_adhoc("Condition failed: `1 < 2`"));
}