Fix "lambda in object literal" missing apply() in vtable.

[redvers]

The old code in reify_typeparamref() followed the ast_data chain only to one level. When they don't match, the type parameters are not substituted during subtype checking causing is_subtype() to incorrectly return false. In this case, the lambda's apply() function isn't marked as reachable resulting in a missing vtable entry and consequent runtime segfault.

Fixes: #4838

[ponylang-main]

Hi @redvers,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 4840.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.