[foundation] Cache parts of NSObject.ConformsToProtocol

Note that the call to native code still _always_ happen (not cached) since the application could use `class_addProtocol` to add conformance to a protocol at runtime.

So the cache is limited to the .net specific reflection code that is present (only) when the dynamic registrar is included inside applications. This is the default for macOS apps, but not iOS / tvOS or MacCatalyst apps.

The linker/trimmer will remove the caching code when the dynamic registrar is removed. IOW this PR should not have any impact, performance or size, for most iOS apps (where the dynamic registrar is removed by default).

Fix xamarin#14065

Running Dope on macOS, a 2 minutes benchmark, shows the following times (in seconds and percentage) spent calling this API:

**Before**

* `RemoveFromSuperview` 7.99s (6.4%)
  * `NSObject.ConformsToProtocol` 3.26s (2.6%)

**After**

* `RemoveFromSuperview` 4.67s (3.8%)
  * `NSObject.ConformsToProtocol` 0.32s (.26%)

So a 10x improvements on `ConformsToProtocol` which helps a lot the code path calling `RemoveFromSuperview`.

src/Foundation/NSObject2.cs

@@ -475,6 +475,36 @@ public virtual bool ConformsToProtocol (NativeHandle protocol)
 			if (!Runtime.DynamicRegistrationSupported)
 				return false;
 
+			// the linker/trimmer will remove the following code if the dynamic registrar is removed from the app
+			var classHandle = ClassHandle;
+			bool new_map = false;
+			lock (protocol_cache) {
+#if NET
+				ref var map = ref CollectionsMarshal.GetValueRefOrAddDefault (protocol_cache, classHandle, out var exists);
+				if (!exists)
+					map = new ();
+				ref var result = ref CollectionsMarshal.GetValueRefOrAddDefault (map, protocol, out exists);
+				if (!exists)
+					result = DynamicConformsToProtocol (protocol);
+#else
+				if (!protocol_cache.TryGetValue (classHandle, out var map)) {
+					map = new ();
+					new_map = true;
+					protocol_cache.Add (classHandle, map);
+				}
+				if (new_map || !map.TryGetValue (protocol, out var result)) {
+					result = DynamicConformsToProtocol (protocol);
+					map.Add (protocol, result);
+				}
+#endif
+				return result;
+			}
+		}
+
+		static Dictionary<NativeHandle, Dictionary<NativeHandle, bool>> protocol_cache = new ();
+
+		bool DynamicConformsToProtocol (NativeHandle protocol)
+		{
 			object [] adoptedProtocols = GetType ().GetCustomAttributes (typeof (AdoptsAttribute), true);
 			foreach (AdoptsAttribute adopts in adoptedProtocols){
 				if (adopts.ProtocolHandle == protocol)