Upgrade to C# 10, Code Analyzers about ontopic-library HOT 1 CLOSED

As part of this, I took advantage of the following new(er) C# capabilities:

• Modified code to use natural types for generics; this was especially useful for XUNit's Assert.Equal<>() overloads, which no longer need the type parameter to be set explicitly; these can be inferred (6aca48c).
• Updated type checking statements to prefer is and is not where practical. This replaced a handful of cases where we were previously using Equals(typeof()) or even IsAssignableFrom() (e6bd2b7, 5852d10).
• Implemented new C# 10 constant string interpolation for [Obsolete()] attributes, which allows (most) class and member references to use nameof() instead of being hardcoded as strings (427959b).
• Fixed a handful of new issues identified by the new code analyzers while I was at it (89900d4, 32f85b9, 995bba9, &c.).

See 5eaa417 for details.

from ontopic-library.