Reduce memory usage for rune width cache.

[Pannoniae]

• 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
• All newly added code is adequately covered by tests
• All existing tests are still running without errors
• 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 👍 this pull request if you are interested in it.

[Pannoniae]

@microsoft-github-policy-service agree

[phil-scott-78]

I think I was the last to touch this an it looks good to me. Pulled it down and tested it, all good. Slightly faster. Only thing I'd be tempted to change would be using Array.Fill instead of the loop to grab some perf since that's the goal here, but we'd need a polyfill or a conditional for a fall back to the loop for netstandard 2.0

[Pannoniae]

Done