…custom System.nint/System.nuint.

Unfortunately "using X = nint;" doesn't work, so we'll have to use the underlying
type for nint instead in the using statement.

…e for NSNumber conversions.

…thods.

…Ptr parameters in .NET.

In legacy Xamarin, the following code would create an NSNumber:

    NSObject.FromObject ((nint) 123);

and this would create an NSValue:

    NSObject.FromObject ((IntPtr) 123);

In .NET, these two types are the same, so we can't distinguish between them. However, we now have ObjCRuntime.NativeHandle that represents handles, and we're already returning an NSValue for that case:

    NSObject.FromObject ((NativeHandle) (IntPtr) 123);

which means that we can make both of these cases return an NSNumber (for .NET):

    NSObject.FromObject ((nint) 123);
    NSObject.FromObject ((IntPtr) 123);

Fixes this test failure:

    MonoTouchFixtures.Foundation.NSObjectTest
     [FAIL] FromObject_NativeTypes : System.InvalidCastException : Unable to cast object of type 'Foundation.NSValue' to type 'Foundation.NSNumber'.
            at MonoTouchFixtures.Foundation.NSObjectTest.FromObject_NativeTypes() in xamarin-macios/tests/monotouch-test/Foundation/ObjectTest.cs:line 111

which happens with this code:

    var nativeint = (NSNumber) NSObject.FromObject ((nint)(-42));