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

[spouliot]

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

Example (array)

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