[RFC] Bind pointers to structs in parameters

Currently bgen doesn't support binding pointers to structs in api definitions.

Example:

```objc
(void) doSomething: (struct SomeStruct* ) value withNumber: (NSInteger) someNumber;
```

This can mean:

* `value` is a pointer to a single value of `struct SomeStruct`.
* `value` is an array of `struct SomeStruct` elements, the number of elements specified by some other means:
* Another parameter (`someNumber` for instance)
* Terminated by some sentinel value
* A field/property on the type.
* Convention / documentation.
* Anything else?

In any case, may be NULL if `__nullable`.

Bind as any of:

No length (single struct):

```cs
[Export ("doSomething:withNumber:")]
unsafe void DoSomething ([PointerAsStruct] SomeStruct* value, nint someNumber);
unsafe void DoSomething ([PointerAsStruct] ref SomeStruct value, nint someNumber);
unsafe void DoSomething ([PointerAsStruct] out SomeStruct value, nint someNumber);
```

Length specified as a parameter:

```cs
unsafe void DoSomething ([PointerAsArray (LengthFromParameter = nameof (someNumber))] SomeStruct[] value, nint someNumber);
```

Length specified as a member:

```cs
unsafe void DoSomething ([PointerAsArray (LengthFromMember = nameof (PropertyOrField))] SomeStruct[] value, nint someNumber);
```

Length specified with convention:

```cs
unsafe void DoSomething ([PointerAsArray (Length = 3] SomeStruct[] value, nint someNumber);
```

Unsupported:
* Length is specified in ways not explicitly supported (examples?)