Add support for objective state, observable/optional members to corto_rw API

[SanderMertens]

Currently the corto_rw API does not correctly handle observable/optional fields and fields of a target type (objective state). The corto_rw API should:

• set the right member of a target member depending on whether the thread owns the object
• correctly set the value of observable members (requires extra dereference)
• correctly set the value of optional members (requires extra dereference) and add feature to unset optional value

For target members, the API should automatically pick the target or actual members when not explicitly specified. For example, given the following type description:

struct Foo {
    m: target[int32]
}

and the following code:

corto_object o = corto_create(data_o, "my_obj", Foo_o);
corto_rw rw = corto_rw_init(Foo_o, o);
corto_rw_field(&rw, "m");
corto_rw_set_int(&rw, 10);
corto_rw_deinit(&rw);

if my_obj is owned by the current thread, the m.actual member should be set. If the object is not owned, the m.target member should be set. The target, actual and objective members may also be explicitly set. When attempting to explicitly assign a member that is not owned by the thread, the set operation will fail.

The API should also take into account complex types. Take for example this type definition:

struct Point {
    x, y: int32
}
struct Foo {
    m: target[Point]
}

It should be possible to select field expression m.x, and depending on whether the object is owned or not, the API will set m.target.x or m.actual.x.