What does this implement/fix? Explain your changes.

Since .NET strings are currently treated as "primitive" types (as they can not be constructed uninitialised, like other non-primitive types can), we only provide the "simple" constructor that converts a Python string or copies an existing String instance.

Does this close any currently open issues?

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Any other comments?

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Checklist

Check all those that are applicable and complete.

• Make sure to include one or more tests for your change
• If an enhancement PR, please create docs and at best an example
• Ensure you have signed the .NET Foundation CLA
• Add yourself to AUTHORS
• Updated the CHANGELOG