Remove syn dependency

[mmastrac]

This adds some extra compile time that we really don't need.

[mitsuhiko]

I did some experiments with ctor stuff recently and ended up implementing just the #[ctor] attribute here without using syn. Feel free to use this: https://crates.io/crates/small_ctor

That version also makes unsafe mandatory (also proposed by #159).

[mmastrac]

@mitsuhiko Excellent! If you'd like, I'm happy to pull this code into the repo. We can add you as a contributor as well.

[mitsuhiko]

I'm happy to contribute. I mostly ended up with having this separate for now because I didn't feel enough motivation to touch all the other parts of it yet (static, dtor) which I do not need.