Rollup merge of #37688 - eddyb:lazy-8, r=petrochenkov

[8/n] rustc: clean up lookup_item_type and remove TypeScheme.

_This is part of a series ([prev](https://github.com/rust-lang/rust/pull/37676) | [next]()) of patches designed to rework rustc into an out-of-order on-demand pipeline model for both better feature support (e.g. [MIR-based](https://github.com/solson/miri) early constant evaluation) and incremental execution of compiler passes (e.g. type-checking), with beneficial consequences to IDE support as well.
If any motivation is unclear, please ask for additional PR description clarifications or code comments._

<hr>

* `tcx.tcache` -> `tcx.item_types`
* `TypeScheme` (grouping `Ty` and `ty::Generics`) is removed
* `tcx.item_types` entries no longer duplicated in `tcx.tables.node_types`
* `tcx.lookup_item_type(def_id).ty` -> `tcx.item_type(def_id)`
* `tcx.lookup_item_type(def_id).generics` -> `tcx.item_generics(def_id)`
* `tcx.lookup_generics(def_id)` -> `tcx.item_generics(def_id)`
* `tcx.lookup_{super_,}predicates(def_id)` -> `tcx.item_{super_,}predicates(def_id)`

src/librustc_metadata/decoder.rs

@@ -527,7 +527,7 @@ impl<'a, 'tcx> CrateMetadata {
 
         ty::TraitDef::new(data.unsafety,
                           data.paren_sugar,
-                          tcx.lookup_generics(self.local_def_id(item_id)),
+                          tcx.item_generics(self.local_def_id(item_id)),
                           data.trait_ref.decode((self, tcx)),
                           self.def_path(item_id).unwrap().deterministic_hash(tcx))
     }