Rustdoc cross-crate search should list items from the current crate first

[huonw]

Things from other crates can be explicitly divided off, e.g. inside std, searching for repr is currently hard to interpret, but could look like

• std::repr
• std::raw::Repr::repr
• std::raw::Repr
• std::repr::Repr
• std::repr::repr_to_str
• std::repr::ReprVisitor

---

Other crates

• rustc::middle::trans::adt::Repr
• rustc::middle::borrowck::RestrictionSet::repr
• ...

This is especially important with large crates like rustc and syntax being included, which are full of unnecessarily exported things, and are rarely what users want.

(@cmr and @chrismorgan: I saw you discussing this on IRC, but I don't think either of you filed a bug, close as a dupe if you did.)