[Generator] Add a smarter generation when [NullAllowed] and [BindAs (..)] are combined

We have APIs that do point to a NSNumber that can be null and we use bind as, for example, we find cases were we have to write:

```csharp
[NullAllowed]
[BindAs (typof (int?)]
NSNumber Foo { get; set; }
```

This is at least, error prone, since we can have cases in which we do by accident:

```csharp
[NullAllowed]
[BindAs (typof (int)]
NSNumber Foo { get; set; }
```

The error is hard to notice while writing the code or doing a review since it is a single character. We should:

1. Teach the generator to raise a warning with the mismatch.
2. Make BindAs smarter, do we need the ? because we already have NullAllowed, just do it.