rustdoc search: rank identity functions lower unless requested

[lolbinarycat]

whenever you perform a -> SomeType query, two results almost always show up in the top 5 results: SomeType::clone, and the reflexive SomeType::from.

when people perform this type of search, they are almost always doing it because they need a value of that type, and don't already have one.

I think that when there are no input arguments in a type-based search, we should rank items that take their return type as an argument below those that don't. if repeatedly checking the arguments of functions during sort has significant performance impact, we can amortize it by caching this property in the search index.