Find item in container using the container's test
method for comparisons. The test method must take two parameters. The
first will be the item being searched for; the second will be an item
in the container. If the container has a key (keyed-container-mixin),
then the test is performed on the item and the key of each element
in the container. Find-item returns nil if the item is not found and it
returns the element in the container if it is.