Add Rust plugin APIs

[mbrobbel]

This proposal is work-in-progress.

To enable ergonomic development of plugins with Rust, we should add some feature-gated APIs with common methods and structures.

My proposal:

• Add a plugin feature which is not part of the default features. Plugins can then depend on dqcsim like this:

[dependencies]
dqcsim = { version = "..", default-features = false, features = ["plugin"] }

• Move the dqcsim::common::gates module behind the plugin feature-gate.
• Add FrontEnd//Backend/Operator traits:

trait FrontEnd {
    fn run(&self)
    fn initialize(&self) -> { .. }
    fn host(&self) -> { .. }
}

• Add a (procedural macro) to derive a PluginDefinition (PluginType and PluginMetadata) for a struct. Also allow deriving the plugin arguments (opts).

#[derive(FrontEnd)]
#[dqcsim(name="front-end", author = "me", version = "0.1.0")]
struct MyPlugin {
  //#[dqcsim(opt="input.file")]
  input: PathBuf,
}

#[derive(DqcsimOpt)]
struct MyPluginOpt {
}

impl TryFrom<Vec<ArbCmd>> for MyPluginOpt {}

impl MyPlugin {
  #[dqcsim(run)]
  fn run(&self, opt) {
      let opt: MyPluginOpt = opt.try_from()?;

  }
}

//
impl FrontEnd for MyPlugin {
  fn run(&self) -> {
    MyPlugin::run(self)
  }
}