[cpp/chapter4]: Added folder for chapter4 & file for a1
[cpp/chapter4]: a5 added
[cpp/chapter4]: a8 added
[cpp/chapter4]: a1 - Added intro comment
[cpp/chapter4] a8: Capitalize stack name & make STACK_SIZE signed

STACK_SIZE made signed to supress a warning
[cpp/chapter4]: a8: Stack implemented using linked list
[cpp/chapter4]: a18 added
[cpp/chapter4]: a8: stack capitalization was fixed, so removed typedef
[cpp/chapter5]: a4 added (after creating folder of chapter5)