change increment argument in nasal_gc, because new vm doesn't need to alloc new objects frequently.

vm_str  2048 -> 512

vm_func 1024 -> 512

vm_vec  8192 -> 512

nasal_gc.h

@@ -29,9 +29,9 @@ const uint32_t increment[vm_type_size]=
     0,   // vm_nil
     0,   // vm_num
     /* gc object */
-    2048, // vm_str
-    1024, // vm_func
-    8192, // vm_vec
+    512, // vm_str
+    512, // vm_func
+    512, // vm_vec
     512, // vm_hash
     64   // vm_obj
 };