Refactor HIR item-like traversal

[cjgillot]

HIR is globally traversed using the tcx.hir().{,par_}visit_all_item_likes{,_in_module} family of functions. These functions create a lot of dependency edges (they call the hir_crate query, or call hir_owner for each item-like). These dependencies are not always useful: many use sites only require items, and don't care for impl items, trait items or foreign items.

This can be avoided by replacing those functions by a scheme based on ItemIds. Actual user will be responsible for calling tcx.hir().item to get the HIR. The amount of access to HIR will be further reduced by pre-filtering using tcx.def_kind query.

Steps:

• create a hir_crate_items query which traverses tcx.hir_crate(()).owners to collect all the ids and return a rustc_middle::hir::ModuleItems;
• use tcx.hir_crate_items in tcx.hir().items to return an iterator of hir::ItemId;
• use tcx.hir_crate_items to introduce a tcx.hir().par_items(impl Fn(hir::ItemId)) to traverse all items in parallel;
• gradually replace uses of tcx.hir().{,par_}visit_all_item_likes with tcx.hir().par_items or tcx.hir().items;
  when possible, create a fast path using tcx.def_kind(item_id.def_id) before calling tcx.hir().item(item_id);
• introduce tcx.hir().items_in_module and tcx.hir().par_items_in_module which do the same thing with tcx.hir_module_items instead of tcx.hir_crate_items;
• gradually replace uses of tcx.hir().visit_all_item_likes_in_module with tcx.hir().par_items_in_module or tcx.hir().items_in_module;
• retire hir::ItemLikeVisitor.

The steps starting with "gradually" can be split into several PRs.
I am available on zulip for any question.

[kckeiks]

@rustbot claim

[kckeiks]

Not sure why rustbot is ignoring me but I'd like to pick this up. I'll have some bandwidth in the next couple of weeks to work on this.