`Fiddle::Pointer#to_value` is unsafe

Well, it's pretty self-explanatory. It returns (VALUE)(data->ptr), and ptr is obviously not marked, so can be GCed. This method is either should be removed completely (it'll still e possible to hack around with Fiddle.dlunwrap, but this at least feels hacky) or to be allowed only for pointers marked to be in fact VALUEs (mark return value type in Function constructor), and they of course should gc_mark their ptrs. Btw, there is no way to create a pointer from Ruby object, either.

首先阅读 Fiddle::Pointer#to_value 的实现和 Function 构造函数，然后检查指针标记以及 Fiddle.dlunwrap 如何与 Ruby 的垃圾回收器交互。该 issue 提出了移除和类型限制两种替代方案；完成此 issue 需要确定安全性设计、完成实现，并覆盖受影响的行为。