Replace type variables in a single sweep during reification.

[SeanTAllen]

Fixes #1875

[ponylang-main]

Hi @SeanTAllen,

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 3664.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.

[SeanTAllen]

@jemc I'm not at all good with the compiler. Do you have any insights on fixing the current assertion failure?

[SeanTAllen]

@jemc ping.

[SeanTAllen]

@jemc says he remembers (he thinks) that in subtype.c there's logic to handle two types that have the same constraints, they might not be super or subtype without knowing more. For example #share. One could be val another could be tag.

Post reification we can check the val/tag in subtype.c. And there's special logic for it there. Joe's guess is that logic is know broken.

This would require someone to look into this some more. Perhaps Joe or Sean.