This change effectively backs out f1737ddd06,
implementing the same functionality in a simpler and more correct way.
Implementation details:
* imv_navigator.wrapped - a new field that has value "0" by default; once
navigator wraps around the list, this field recieves value "1".
* imv_navigator_wrapped(&nav) - a new function that returns non-zero value iff
navigator wrapped around the list. Currently just returns value of
imv_navigator.wrapped.
While at it, expanded "test_navigator_remove" and merged it with
"test_navigator_add". Resulting test is called "test_navigator_add_remove".
Fixes#94.
