Add annotations to System.Convert

For instance Convert.ToString(string) doesn't do anything and can be annotated as:

```cs
[ContractAnnotation("s: notnull => notnull; s: null => null")]
public static string ToString(string s); // { return s; }
```

Similar to other methods of [Convert](https://docs.microsoft.com/en-us/dotnet/api/system.convert?view=netframework-4.7.2).