Improve const generics parsing

- Handle const generics type args
- Fix issue with const generic as first parameter in trait impl

crates/ra_parser/src/syntax_kind/generated.rs

@@ -234,6 +234,7 @@ pub enum SyntaxKind {
     LIFETIME_ARG,
     TYPE_ARG,
     ASSOC_TYPE_ARG,
+    CONST_ARG,
     PARAM_LIST,
     PARAM,
     SELF_PARAM,