Find node containing thing 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 thing is not found and it
returns the node in the container if it is. Find-node is the same as find-element
for containers that do not use nodes.