C# 13: Allows ref struct.

[michaelnebel]

In this PR we introduce support for the allows ref struct type parameter constraint. The language feature is described here.

A couple of notes on the implementation.

• A ref struct type can not be implicitly converted to a dynamic type, object or ValueType.
• The allows ref struct is a negative constraint meaning that it extends the number of types that can be used as type replacement for a type parameter.

The unification and dispatch call logic has been adapted to take the allows ref struct constraint into account (relevant for dynamic dispatch) for deciding relevant dispatch targets. To make things easier a new class RefStruct has been introduced in the type hierarcy. One could consider whether we want to change the type hierarchy such that RefStruct doesn't extend Struct (and thereby ValueType). Not sure whether this is worth it.

Furthermore, we also extract the notnull general type parameter constraint.