UnicodeUtil updates: TryUTF8toUTF16, ReadOnlySpan methods, #1024

[paulirwin]

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Various improvements to the UnicodeUtil APIs

Fixes #1024

Description

This PR includes several changes to the UnicodeUtil APIs:

• New TryUTF8toUTF16 method added that uses the Try-method pattern common in .NET, so that it will return false if there is an incomplete UTF-8 byte sequence at the end
• UTF8toUTF16 has been updated to throw a ParseException instead of IndexOutOfRangeException if there is an incomplete UTF-8 byte sequence at the end
• New UTF8toUTF16WithFallback method that is like UTF8toUTF16 but adds the U+FFFD character to the output if there is an incomplete UTF-8 byte sequence at the end, instead of throwing
• Moved ToCharArray to the ObsoleteAPI folder as a partial class, and marked for removal in the 4.8.0 RC
• Add ReadOnlySpan overloads where applicable
• Add TryUtf8ToString and Utf8ToStringWithFallback methods on BytesRef that call the respective methods in UnicodeUtil
• Update callers where it makes sense to use these methods, such as ToString and logging/exception formatting where it doesn't make sense to throw an exception on invalid UTF-8

[paulirwin]

Publishing as ready for review as I am not aware of any other usages that need to be updated. Let me know if you find any.