Added init function

Issue : Init/create function

stack.c

 #include "stack.h"
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#define DEFAULT_CAPACITY 4
+
+void stack_init(stack *s)
+{
+    s->top = -1;
+    s->capacity = DEFAULT_CAPACITY;
+    s->data = malloc(sizeof(int) * DEFAULT_CAPACITY);
+}
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