Implement beginnings of generics

- add HIR for generic params
 - resolve generic params in type paths
 - add substitions for ADTs
 - insert type variables for substitutions

crates/ra_ide_api/src/db.rs

@@ -129,6 +129,7 @@ salsa::database_storage! {
             fn body_hir() for hir::db::BodyHirQuery;
             fn body_syntax_mapping() for hir::db::BodySyntaxMappingQuery;
             fn fn_signature() for hir::db::FnSignatureQuery;
+            fn generics() for hir::db::GenericsQuery;
         }
     }
 }