eet-cxx: add implementation for eet C++.

eet-cxx: add implementation for eet C++.

Usage example:

  struct type
  {
    int foo;
    float bar;
  };

  type t0;

  auto descriptor = make_descriptor("type", &type::ofo, &type::bar);

  eet_data_write(file, descriptor.native_handle(), "type", &t0, false);

  std::unique_ptr<type> p = read_by_ptr(file, "type", descriptor);
  type t = read(file, "type", descriptor);

@feature

Reviewers: cedric, smohanty

Reviewed By: cedric

CC: savio, cedric
Differential Revision: https://phab.enlightenment.org/D659

Signed-off-by: Cedric BAIL <cedric.bail@free.fr>