Linked issue

Partly addresses #6589 along with PR #6603.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• Performance improvement (includes benchmark results below)
• Documentation update
• New feature or public API (requires prior proposal or issue approval)
• Refactor / internal cleanup (no user-visible change)
• Build, CI, or tooling change

Motivation

Discussed in #6589.

What changed

Added the once iterator, that yields an element exactly once. For instance

from std.iter import once
from std.testing import assert_equal, assert_raises

var it = once(10)
assert_equal(next(it), 10)
with assert_raises():
   _ = next(it)

Testing

Added new tests and successfully ran the full test suite.

Checklist

• The linked issue above has been reviewed by a maintainer and is
  agreed-upon, or this is a trivial fix that does not need prior
  approval
• PR is small and focused — I've split larger changes into a sequence of
  smaller PRs where possible (see
  pull request sizes)
• I ran ./bazelw run format to format my changes
• I added or updated tests to cover my changes
• If AI tools assisted with this contribution, I have included an
  Assisted-by: trailer in my commit message or this PR description (see
  AI Tool Use Policy)