When working with a non-unique secondary index, one may need a primary
key as well. We have cmp_def, which is key_def merged with pk_def, but
it's difficult to work with primary key in such form. For example, we have
to get primary key definition right from primary index of space to extract
position of tuple in functional index. Let's add primary key definition to
index_def to simplify use of primary key parts and avoid unnecessary
dependencies on space and its primary index.

Part of tarantool/tarantool-ee#285

NO_TEST=no behavior changes
NO_CHANGELOG=no behavior changes
NO_DOC=no behavior changes