Support for the new Utf8String

[roji]

Support reading and writing of the new Utf8String.

Some experimental work is available in https://github.com/roji/Npgsql/tree/Utf8String using System.Text.Utf8String.

[roji]

No confirmation yet that this type is getting into 3.0, or even a consensus that it would be a good idea: dotnet/coreclr#23209 (comment)

[roji]

This appears to have been punted on the .NET Core side, moving to backlog.

[NinoFloris]

Closing this issue. Now that u8 literals have been added to C# there is zero desire left to bifurcate the BCL with two different string types.