Plugged mem leak in vector destructor.
Privatizes the linked list implementation.

Following changes applied:
* Only two files needs compilation - all logic moved into lpriq.c and list.c
* Extended the double ended queue so that it also supports pops directly.
* Privatized the linked list node - no longer directly accessable - supports
only provided functions
* Merged implementation of shared features for deque and linked stack.
Fixed mem leak in heap pop.