Make heap small chunk size setting logic more precise/correct

[dipinhora]

Prior to this commit, the heap small chunk size setting logic would bitwise-or in the size into the appropriate bits of the small chunk but this is technically incorrect as it doesn't clear out the bits first.

This commit changes the logic to ensure the relevant bits are cleared first before the new size is bitwise-or'd in.

[SeanTAllen]

@dipinhora I believe this would count as a bug fix, yes? And I think, should get a changelog entry and release notes. Do you agree?

[dipinhora]

@dipinhora I believe this would count as a bug fix, yes? And I think, should get a changelog entry and release notes. Do you agree?

@SeanTAllen probably, yes.. in the current implementation the size is only ever set once on a newly allocated small_chunk_t which i believe wouldn't be an issue unless the pool is recycling previously used memory that has those bits already set (unsure how likely that is or if it has already caused issues in the wild)..

[ponylang-main]

Hi @dipinhora,

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 4527.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.

[dipinhora]

release notes added.. feel free to tweak them as needed (or let me know what needs tweaking)..