Eo: Add eo_do_part.

Eo: Add eo_do_part.

This is a convenience macro to be used by the common pattern of getting
a part and then immediately calling functions on it. For example,
  without this macro, you'd have to write code like:

Eo *part;
eo_do(obj, part = efl_part_name_get("partname"));
eo_do(part, a_set(7));

while using the helper function trims it to:

eo_do_part(obj, efl_part_name_get("partname"), a_set(7));

@feature