Complete support for [BindAs] (#2220)

* [registrar] BindAs uses Nullable types so allow them to be registered as NSObjects

BindAsAttribute allows to bind NSValue and NSNumber into more
accurate C# types lyke bool?, int? etc. so we must teach registrar
about this.

* [tests][introspection] Teach intro about BindAs and Nullable types

Introspection will currently fail if BindAs is used, introspection
will report that the incorrect type is registered so we need to skip
this check if Nullable type is found in the signature

* [introspection] Add better type checking instead of totally skipping the type when Nullable type is encountered

Introspection will currently fail if BindAs is used. Introspection
will report that the incorrect type is registered so we need verify
if a Nullable type is found in the signature and check against of
a withelist of BindAs supported types

* Revert "[registrar] BindAs uses Nullable types so allow them to be registered as NSObjects"

This reverts commit 911eab97b7.

* [tests] Add comment about where to find BindAs types

tests/introspection/ApiSignatureTest.cs

@@ -645,6 +645,12 @@ namespace Introspection {
 		{
 			switch (encodedType) {
 			case '@':
+				// We use BindAsAttribute to wrap NSNumber/NSValue into more accurate Nullable<T> types
+				// So we check if T of nullable is supported by bindAs
+				var nullableType = Nullable.GetUnderlyingType (type);
+				if (nullableType != null)
+					return BindAsSupportedTypes.Contains (nullableType.Name);
+
 				return (type.IsInterface ||								// protocol
 					type.IsArray || 									// NSArray
 					(type.Name == "NSArray") || 						// NSArray
@@ -980,5 +986,14 @@ namespace Introspection {
 			}
 			return false;
 		}
+
+		// This must be kept in sync with generator.cs NSValueCreateMap and NSValueReturnMap
+		protected HashSet<string> BindAsSupportedTypes = new HashSet<string> {
+			"CGAffineTransform", "Range", "CGVector", "SCNMatrix4", "CLLocationCoordinate2D",
+			"SCNVector3", "Vector", "CGPoint", "CGRect", "CGSize", "UIEdgeInsets",
+			"UIOffset", "MKCoordinateSpan", "CMTimeRange", "CMTime", "CMTimeMapping",
+			"CATransform3D", "Boolean", "Byte", "Double", "Float", "Int16", "Int32",
+			"Int64", "SByte", "UInt16", "UInt32", "UInt64", "nfloat", "nint", "nuint",
+		};
 	}
 }