csharp: Add IntPtr to/from IEnumerable conversion for "accessor" types

csharp: Add IntPtr to/from IEnumerable conversion for "accessor" types

Adds a special case for "accessor" complex types in `implicit operator` for
structs and `IntPtr`s, in which an IEnumerator must be converted to/from an
IntPtr.

Reviewed-by: YeongJong Lee <cleanlyj@naver.com>
Reviewed-by: Felipe Magno de Almeida <felipe@expertisesolutions.com.br>
Differential Revision: https://phab.enlightenment.org/D11210