Upgrade library to C# 7.3

[NN---]

This allows using enum constraint:
https://blog.jetbrains.com/dotnet/2018/07/19/unmanaged-delegate-enum-type-constraints-c-7-3-rider-resharper/

[theraot]

Code is now C# 8.0 - however, I do not think I will be using the enum contraint. Not for HasFlag or TryParse, they will keep close to the signature in the BCL.