Use HashStable_Generic in rustc_type_ir

A lot of the types in this crate implemented HashStable directly to
avoid circular dependencies. One way around that is to use
HashStable_Generic. We adopt that here to avoid a lot of boilerplate.

This doesn't update all the types, because some would require
`I: Interner + HashStable`.

compiler/rustc_query_system/src/ich/impls_syntax.rs

@@ -148,3 +148,5 @@ impl<'tcx> HashStable<StableHashingContext<'tcx>> for rustc_feature::Features {
         });
     }
 }
+
+impl<'ctx> rustc_type_ir::HashStableContext for StableHashingContext<'ctx> {}