An ItemLocalId uniquely identifies something within a given “item-like”;
that is, within a hir::Item, hir::TraitItem, or hir::ImplItem. There is no
guarantee that the numerical value of a given ItemLocalId corresponds to
the node’s position within the owning item in any way, but there is a
guarantee that the ItemLocalIds within an owner occupy a dense range of
integers starting at zero, so a mapping that maps all or most nodes within
an “item-like” to something else can be implemented by a Vec instead of a
tree or hash map.