Use built-in offset argument to cpointer

[ergl]

No description provided.

[ergl]

@SeanTAllen should this have release notes for the possible memory leak?

[SeanTAllen]

@ergl yes

[ponylang-main]

Hi @ergl,

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 3741.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.