Reduce memory usage for rune width cache.

[Pannoniae]

• [X] I have read the Contribution Guidelines
• [ ] I have commented on the issue above and discussed the intended changes
• [ ] A maintainer has signed off on the changes and the issue was assigned to me
• [X] All newly added code is adequately covered by tests
• [X] All existing tests are still running without errors
• [X] The documentation was modified to reflect the changes OR no documentation changes are required.

Changes

The rune width cache array consumes half a megabyte because it uses a Nullable to store values. This PR slashes the memory requirements to an eighth of what it was before with no loss of functionality.

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Please upvote :+1: this pull request if you are interested in it.