Remove two dependencies

[FreezyLemon]

Remove thiserror and paste. Both are proc-macros, but syn is still required because of av-data using num-derive in a few places.

Still, I think these are simple changes (without API breakage) and fewer dependencies are good.

[shssoichiro]

I have mixed feelings about this, because of the increased boilerplate. Let me think about it... or convince me. 🤷‍♂️

[FreezyLemon]

You mean because the #[error("...")] syntax is nicer than the manual Error implementation? I can see that.. I've made it a habit to avoid proc-macros unless they pull their weight. Maybe it was overzealous in this case, because we can't avoid syn anyways with av-data still pulling it in. And I've gotten used to the boilerplate, I guess.

I don't really have a strong stance on this. If you think it's not worth it, that's fine.

[shssoichiro]

That as well as the duplication from removing paste. What impact does it have on the compilation time? Maybe not as much as we'd prefer since we still need syn at the moment.

[FreezyLemon]

A fresh build on my notebook is a tiny bit quicker (0.5s / 10%)...

I tried replacing num-derive (last syn-based dependency) in av-data a few months ago, but that was understandably rejected. The relevant commit is still accessible, but as you can see, the code for that is an arguable downgrade. I guess we may need to wait for something like rust-lang/rfcs#3698

[shssoichiro]

We may be waiting a long time for that. In the meantime, I suppose this change isn't that bad, since it's just a small amount of boilerplate.