c/ffi: add _demo

c/ffi/_demo/_wrap/wrap.c

@@ -0,0 +1,25 @@
+#include <stdio.h>
+
+struct array
+{
+    int x;
+    int y;
+    int z;
+    int k;
+};
+
+int demo1(struct array a)
+{
+    printf("c.demo1: %d %d %d %d\n",a.x,a.y,a.z,a.k);
+    return a.x+a.y+a.z+a.k;
+}
+
+int demo2( int (*fn)(struct array)) {
+    printf("c.demo2: %p\n",fn);
+    struct array a;
+    a.x = 1;
+    a.y = 2;
+    a.z = 3;
+    a.k = 4;
+    return (*fn)(a);
+}