Resolve "Add peek function"

Closes #7 (closed)

stack.c

-#include "stack.h"
-#include <assert.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include "stack.h"
 
 #define DEFAULT_CAPACITY 4
 
@@ -10,4 +9,10 @@ void stack_init(stack *s)
     s->top = -1;
     s->capacity = DEFAULT_CAPACITY;
     s->data = malloc(sizeof(int) * DEFAULT_CAPACITY);
-}
+}
\ No newline at end of file
+
+void stack_peek(stack s, int *value){
+    if (!stack_is_empty(s)) {
+        *value = s.data[s.top];
+    }
+}