Account for trait implementations and other kinds of deep searching

The essential problem is that this search query doesn't work the way it would be expected to:

Iterator<T>, (Fn(T) -> U) -> Iterator<U>

That sure looks like it would be the type signature of Iterator::map, but, technically, it's not. That's Iterator<T>, F -> Map<F>, because there's a special concrete type used there.

Iterator<Result<T>> -> Result<T>

Similarly, this looks like it would be Result::from_iter, but, again, it's not. It's technically IntoIterator<Result<T>> -> Result<T>

To make the search actually work, we need to account for trait implementations. From search.js's point of view, all of this can be treated like subtyping (in the language, that's not technically what it is, but this is a search engine and it can play faster and looser with things).

Start with search.js and reproduce the two example queries involving Iterator::map and Result::from_iter. Trace how type signatures are matched, then verify that searches account for trait implementations as described and return the expected results.