Refactor the HIR so that items are stored in a map in the `Crate`,

Refactor the HIR so that items are stored in a map in the `Crate`,
rather being stored inline. Refactor (and rename) the visitor so that
(by default) it only visits the interior content of an item not nested
items.

This is a [breaking-change] for anyone who uses the HIR visitor. Besides
changing `visit::` to `intravisit::`, you need to refactor your visitor
in one of two ways, depending on what it requires:

1. If you just want to visit all items (most common), you should call
   `krate.visit_all_items(&mut visitor)`.

2. If you need to visit nested items in the middle of the parent items,
   you should override `visit_nested_item` with something like:
   `self.visit_item(self.tcx.map.expect_item(item.id))`, presuming you
   have access to a tcx (or at least a HIR map).