[generator] Remove temporary creation of `NSNumber` instances in generated bindings

Current code generation can create multiple `NSNumber` (alone or in an array) instance that needs to be allocated (and soon after disposed).

## Example (array)

```csharp
IntPtr retvalarrtmp;
ret = ((retvalarrtmp = global::ObjCRuntime.Messaging.IntPtr_objc_msgSendSuper (this.SuperHandle, Selector.GetHandle ("gaugeColorLocations"))) == IntPtr.Zero ? null : (NSArray.ArrayFromHandleFunc (retvalarrtmp, ptr => {
using (var num = Runtime.GetNSObject (ptr)) {
return (float) num.FloatValue;
}
})));
```

A more direct approach would be to use `CFNumberGetValue` [1] to avoid the extra costs.

[1] https://developer.apple.com/documentation/corefoundation/1543114-cfnumbergetvalue?language=objc