Pri 0

• Make encoding between UTF8 and UTF16 (both ways) fast

Pri 1

• Hide System.Text.Utf8 and System.Text.Utf16 namespaces, except for Utf8String
• Add TextEncoder overloads hinted in comments in source.
• Move UnicodeCodePoint to some subnamespace, or hide for now, if not needed by bullet above
• Support other encodings, e.g. 8859-*
• We need ability to continue (as opposed to restart) encoding after output buffer space has been exhausted

Pri 2

• Rename EncodingData to something better
• Rename TextEncoder.Id type, and Scheme property