collation: split collation into coll and id objects

In the issue #3290 the important problem appeared - Tarantool can
not create completely internal collations with no ID, name,
owner. Just for internal usage.

Original struct coll can not be used for this since
* it has fields that are not needed in internals;
* collation name is public thing, and the collation cache uses
  it, so it would be necessary to forbid to a user usage of some
  system names;
* when multiple collations has the same comparator and only their
  names/owners/IDs are different, the separate UCollator objects
  are created, but it would be good to be able to reference a
  single one.

This patch renames coll to coll_id, coll_def to call_id_def and
introduces coll - pure collation object with no any user defined
things.

Needed for #3290.