[BitArray] Disable bitwise operators for now

[lorentey]

The current family of operations is far too restrictive, and the generalization is a topic of ongoing investigation. In particular, the operator syntax seems unlikely to survive when we want to support operations over slices.

Don’t ship BitArray with API that we’ll potentially regret; instead, just leave these off for now.

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [X] I've followed the coding style of the rest of the project.
• [X] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [ ] I've added benchmarks covering new functionality (if appropriate).
• [X] I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• [ ] I've updated the documentation if necessary.

[lorentey]

(The macOS tests are expected to fail due to a CI issue. It does not indicate an issue with these changes.)

@swift-ci test