lib: update and simplify the `bt_object` API

lib: update and simplify the `bt_object` API

This patch merges `ref-internal.h` into `object-internal.h` so as to
simplify the `bt_object` API and ease the future addition of specialized
inline functions (e.g., no specific checks).

Notable changes:

* There's no specific reference count structure; the reference count is
  directly within `struct bt_object`.

* Most functions of `object-internal.h` accept and return the
  `struct bt_object *` type instead of `void *`.

* The `struct bt_object` structure contains two release function:

  `release_func`:
      Called when the reference count falls to zero by the new
  	  bt_object_put_no_null_check() function.

  `spec_release_func`:
      Called by bt_object_try_spec_release() (formerly
  	  bt_object_release()).

* You can initialize a unique object (no reference count) with
  bt_object_init_unique(), a shared object that will _never_ have any
  parent with bt_object_init_shared(), and a shared object which could
  have a parent in the future with bt_object_init_shared_with_parent().

  bt_object_init_shared_with_parent() calls bt_object_init_shared() with
  the special `bt_object_with_parent_release_func` callback as the
  object's release function. This function used to be called
  generic_release(): it puts the parent if there is one, or releases the
  object itself (with bt_object_try_spec_release()) when there's none.

  The whole job of bt_object_with_parent_release_func() is unnecessary
  for shared objects which will never have a parent: in this case, we
  can call a single release function as soon as the object's reference
  count falls to zero. This can render the whole system faster as
  notification objects are shared, but without a parent, for example.
  Packet objects are another example, albeit less significant.

* There are two new, internal versions of bt_get() and bt_put() which do
  not check if the parameter is null: bt_object_get_no_null_check() and
  bt_object_put_no_null_check(). We can use those functions internally
  on the fast path where it is known that a given Babeltrace object
  pointer is never null. This is what the public bt_get() and bt_put()
  function use internally now, after checking some preconditions.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>