Implement "ObserverRef" to avoid explicitly referring to observer types

[addisoncrump]

We saw this here: #1886

Basically, to access an observer, we currently have to explicitly refer to the type of the observer when fetching it with match_name and friends. We can sidestep this by defining an ObserverRef type, e.g.:

#[derive(Copy, Debug, Serialize, Deserialize)]
pub struct ObserverRef<T>(PhantomData<T>);

Then defining (default) in Observer:

pub fn make_reference(&self) -> ObserverRef<Self> {
  ObserverRef(PhantomData)
}

And finally defining a MatchNameRef blanket:

pub trait MatchNameRef {
  fn match_name_by_ref<T>(&self, name: &str, ref: ObserverRef<T>) -> Option<&T>;
}

impl<M> MatchNameRef for M where M: MatchName {
  fn match_name_by_ref<T>(&self, name: &str, ref: ObserverRef<T>) -> Option<&T> {
    self.match_name::<T>(name);
  }
}

In this way we can avoid ever explicitly referring to type names, since the type will be encoded in the ObserverRef wrapper and inferred.

[addisoncrump]

In fact, we have this pattern in a few places (e.g. metadata) so maybe it would be better to define TypeRef and just make it a blanket impl (impl<T> TypeRef for T).

[domenukk]

I think if we're doing this, we can have the ref also include the name, then we have a real 1:1 mapping of observer to the match, right? That'd be amazing