type_info: reflection on non-dyn compatible traits

[khionu]

Right now, if a trait as associated items, reflection is difficult to impossible. My personal case is trying to reflect on an async function. I quickly run into the problem of being unable to determine if a function returns a Future because I have no way of binding Future::Output.

A possible solution that @oli-obk and I brainstormed would be a trait_of! macro, which would take the implementer, plus the trait as an ident, and return a Trait. Further, extensions similar to the below would need to be added around Trait.

pub struct Trait {
  // ...
  pub associated: &'static [TraitAssocProperty];
}

pub struct TraitAssocProperty {
  pub name: &'static str;
  pub kind: TraitAssocPropertyKind;
  pub ty: TypeId,
}

pub enum TraitAssocPropertyKind {
  Constant,
  Type
}

Related to: #146922 and #144361