[perf] Cache Runtime.IsUserType results

This is computed **twice** _per instance_, when the instance is
* created: `NSObject.CreateManagedRef`
* released: `NSObject.ReleaseManagedRef`

However its (boolean) value remains identical for all instances of the same type.

This shows up as consuming a lot of time in the logs attached to dotnet#15145

Basically two things are cached
* the selector for `xamarinSetGCHandle:flags:`, which makes it 15% faster;
	 * some platforms, but not macOS, have an optimization for the `Selector.GetHandle` API
* the result is cached into a `Dictionrary<IntPtr,bool>`

Note that the optimizations are not specific to CoreCLR nor macOS (so they are not fixes for the CoreCLR performance regression of the above mentioned issue).
OTOH it will also help performance for legacy and other net6 (mono) platforms.

```
BenchmarkDotNet=v0.12.1, OS=macOS 12.3.1 (21E258) [Darwin 21.4.0]
Apple M1, 1 CPU, 8 logical and 8 physical cores
.NET Core SDK=        6.0.100 [/usr/local/share/dotnet/sdk]
  [Host] : .NET Core 6.0 (CoreCLR 6.0.522.21309, CoreFX 6.0.522.21309), Arm64 RyuJIT

Job=InProcess  Toolchain=InProcessEmitToolchain  IterationCount=3
LaunchCount=1  WarmupCount=3
```

|           Method | Length |           Mean |        Error |      StdDev | Ratio |
|----------------- |------- |---------------:|-------------:|------------:|------:|
|         Original |     16 |     7,729.8 ns |    212.61 ns |    11.65 ns |  1.00 |
|   CachedSelector |     16 |     6,552.6 ns |    202.70 ns |    11.11 ns |  0.85 |
| CachedIsUserType |     16 |       162.0 ns |     14.86 ns |     0.81 ns |  0.02 |
|                  |        |                |              |             |       |
|         Original |    256 |   123,183.0 ns |  4,724.95 ns |   258.99 ns |  1.00 |
|   CachedSelector |    256 |   104,570.3 ns |  2,029.20 ns |   111.23 ns |  0.85 |
| CachedIsUserType |    256 |     2,489.5 ns |    390.86 ns |    21.42 ns |  0.02 |
|                  |        |                |              |             |       |
|         Original |   4096 | 1,970,381.7 ns | 66,393.09 ns | 3,639.23 ns |  1.00 |
|   CachedSelector |   4096 | 1,676,773.0 ns | 12,149.92 ns |   665.98 ns |  0.85 |
| CachedIsUserType |   4096 |    39,933.3 ns |  7,426.74 ns |   407.08 ns |  0.02 |

[Benchmark source code](https://gist.github.com/spouliot/42fd43e94c5a9ce90164f0f6f9b35018)

src/ObjCRuntime/Runtime.cs

@@ -39,6 +39,7 @@ public partial class Runtime {
 		static List <object> delegates;
 		static List <Assembly> assemblies;
 		static Dictionary <IntPtr, GCHandle> object_map;
+		static Dictionary <IntPtr, bool> usertype_cache;
 		static object lock_obj;
 		static IntPtr NSObjectClass;
 		static bool initialized;
@@ -272,6 +273,7 @@ unsafe static void Initialize (InitializationOptions* options)
 			Runtime.options = options;
 			delegates = new List<object> ();
 			object_map = new Dictionary <IntPtr, GCHandle> (IntPtrEqualityComparer);
+			usertype_cache = new Dictionary <IntPtr, bool> (IntPtrEqualityComparer);
 			intptr_ctor_cache = new Dictionary<Type, ConstructorInfo> (TypeEqualityComparer);
 			intptr_bool_ctor_cache = new Dictionary<Type, ConstructorInfo> (TypeEqualityComparer);
 			lock_obj = new object ();
@@ -1735,11 +1737,23 @@ internal static IntPtr GetProtocolForType (Type type)
 			throw new ArgumentException ($"'{type.FullName}' is an unknown protocol");
 		}
 
-		[BindingImpl (BindingImplOptions.Optimizable)]
 		internal static bool IsUserType (IntPtr self)
 		{
 			var cls = Class.object_getClass (self);
+			if (!usertype_cache.TryGetValue (cls, out var result)) {
+				result = SlowIsUserType (cls);
+				usertype_cache.Add (cls, result);
+			}
+			return result;
+		}
+
+#if __MACOS__
+		static IntPtr selSetGCHandle = Selector.GetHandle ("xamarinSetGCHandle:flags:");
+#endif
 
+		[BindingImpl (BindingImplOptions.Optimizable)]
+		static bool SlowIsUserType (IntPtr cls)
+		{
 			unsafe {
 				if (options->RegistrationMap is not null && options->RegistrationMap->map_count > 0) {
 					var map = options->RegistrationMap->map;
@@ -1752,8 +1766,11 @@ internal static bool IsUserType (IntPtr self)
 						return false;
 				}
 			}
-
+#if __MACOS__
+			return Class.class_getInstanceMethod (cls, selSetGCHandle) != IntPtr.Zero;
+#else
 			return Class.class_getInstanceMethod (cls, Selector.GetHandle ("xamarinSetGCHandle:flags:")) != IntPtr.Zero;
+#endif
 		}
 
 		static unsafe int FindUserTypeIndex (MTClassMap* map, int lo, int hi, IntPtr cls)