perf(utils): speed up and simplify _chunk and _achunk using islice/anext

[vmphase]

Summary

This PR speeds ups and simplifies internal utils._chunk and utils._achunk, which are used by public utils.as_chunks.

Information

• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed).
• This PR is not a code change (e.g. documentation, README, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.
• I have updated the changelog to include these changes.
• I have read the Contributing Guidelines.
• AI Usage has been disclosed.
  • If AI has been used, I understand fully what the code does

[pycord-app]

Thanks for opening this pull request!
Please make sure you have read the Contributing Guidelines and Code of Conduct.

This pull request can be checked-out with:

git fetch origin pull/3342/head:pr-3342
git checkout pr-3342

This pull request can be installed with:

pip install git+https://github.com/Pycord-Development/pycord@refs/pull/3342/head

[vmphase]

@Lulalaby does this need a changelog? Both are rather internal changes.

[Paillat-dev]

If the perf gains are noticeable, the changelog could mention that, otherwise no. In any case it should not specify what changed in the internals. Under ## Changed

[Lulalaby]

Nah

[Paillat-dev]

lgtm