Few ctypes-related todo points:

* Write down missing parts and port all tests, eventually adding
  additional tests.

  - for unions and structs, late assignement of _fields_ is somewhat buggy.
    Tests about behavior of getattr working properly on instances
    are missing or not comprehensive. Some tests are skipped because I didn't
    understand the details.

  - _fields_ can be tuples too as well as lists

  - restype being a function is not working.

  - there are features, which we don't support like buffer() and
    array() protocols.

  - are the _CData_value return lifetime/gc sematics correct?

  - for some ABIs we will need completely filled ffitypes to do the
    right thing for passing structures by value, we are now passing enough
    information to rawffi that it should be possible to construct such precise
    ffitypes in most cases

  - bitfields are not implemented

  - byteorder is not implemented

* as all stuff is applevel, we cannot have it really fast right now.

* we shall at least try to approach ctypes from the point of the jit
  backends (at least on platforms that we support). The thing is that
  we need a lot broader support of jit backends for different argument
  passing in order to do it.