* Passing structs by value is supported with an ugly hack to overcome
  the actual limits of CFFI. We splay args in integer values, so it is
  platform-dependent (only for x86 ABI)

* Use of not exported symbols of CFFI. As ObjC use a different
  convention to push arguments for variadic functions (it doesn't do
  argument promotion), we can't use normal defcfun but we are using
  internal methods of CFFI. We use not exported symbols also to get
  info about foreign types.

* Exception management is missing because we don't know how to catch
  SIGTRAP signals in a portable way

* Circle View example doesn't work