Add opaque struct declaration
Add stack source file
Define struct Stack
Add basic operation function declaration
Add stack creation function declaration
Refine function naming & add declaration
Add stack creation implementation
Add stack empty function implementation
Add stack push implementation
Add stack pop implementation
Add stack destroy implementation

Free allocated memory for stack
Fix incorrect return in pop operation
Include ignore for test & debug source file
Fix missing ; in struct implementation
Add return NULL if popping empty stack