`dlopen_derive` is unmaintained

`dlopen_derive` hasn't been updated since June 9, 2019.

`dlopen_derive` depends on [`quote = "0.6.12"`] and [`syn = "0.15.34"`]. Versions `1.0.0` of these dependencies were published on August 13, 2019. The `0.*` versions haven't received updates since.

Note that `dlopen` is an unmaintained crate from the same repository as `dlopen_derive`. However, migrating away from `dlopen_derive` implies migrating away from `dlopen`, as well.

## Recommended alternatives

- [`dlopen2_derive`] (and `dlopen2`)

[`dlopen2_derive`]: https://github.com/OpenByteDev/dlopen2
[`quote = "0.6.12"`]: https://github.com/dtolnay/quote/releases/tag/0.6.12
[`syn = "0.15.34"`]: https://github.com/dtolnay/syn/releases/tag/0.15.34