Add removeAtIndex(); complete SLList_t.

data_structures/singly_linked_list/src/singly_linked_list.(c, h)
	-Implement and document `removeAtIndex()`.

data_structures/singly_linked_list/src/test.c
	-Revise contents of `test()`: make test-strings allocations
	cleaner by moving all of the pointers to an array, and add calls
	to test `insertAtIndex()` and `removeAtIndex()`.