A few improvement/discussion points of the PSMethod -> Delegate conversion work #5618

This is about #5287

• DelegateArgsComparator.ParameterTypesMatches doesn't consider the Variance in delegate parameter types. A method signature int CallMe(object input) should be able to be converted to Func<string, int>.
• PSMethod.MatchesPSMethodProjectedType doesn't handle the case where both targetType and projectedSourceType are the same ByRef type or Pointer type.
• I think the 'Unit' type name should be changed to 'VOID'. Use type Unit to represent void is not intuitive and I misread it to uint when first looking at the code.
• I don't get the point of having ReplaceGenericTypeArgumentsWithMarkerTypes. After making a generic method using our synthetic reference/value types, the generated MethodInfo signature won't match any real delegates except in one condition -- the original generic method doesn't use the generic type argument for any of its parameters or return type -- I think it's rare for this condition to be met for a generic method.
• I agree that we need pseudo types to represent ByRef type (ref/out), Pointer type void type and even TypeReference, but why do we need PSEnum<> to wrap enum types? Remove PSEnum<> and use the enum type directly.
• Handle method signatures with Span<T> or ReadOnlySpan<T> as its parameter type or return type. They cannot be used as type argument, and therefore we cannot use them directly to construct a generic type of Func<>. In fact, this is a more general problem with all ref-like types, such as public ref struct Foo { private Span<int> _pointer }. It would be great if we can have a generic solution for all ref-like types, but at least we need to handle Span<T> and ReadOnlySpan<T> as they are getting more and more usage. See the discussion comments: Further improve PSMethod to Delegate conversion #6851 (comment)