Added clear
Reimplementation of List (list_rem not tested)

The previous implementation considered only the connections among the nodes
and no container was provided. This approach was risky in that there could
be more than a few dangling nodes with no references and required extra care
in their usage for further development.

The new approach gives a container having head, tail and count. Count is an
important and frequent operation on any list and for a memory footprint of an
extra integer and load of couple of extra instruction (increment and decrement
respectively) while adding & removing nodes, gives a convinient way to traverse
through the list and enumerate the members.