Bug fix: Function pointer can't be used to create a proxy

[mingxwa]

Work item: #49

The root cause of the bug is that std::to_address() does not support function pointers. Per its wording in [pointer.conversion]

template<class T> constexpr T* to_address(T* p) noexcept;
  Mandates: T is not a function type.
  Returns: p.

Checking the history of the proposal P0653R2, this prerequisite was added per LWG review comments:

This is meant only for turning a maybe-smart pointer type to a definitely-raw pointer, if it starts being used for "get address of a function" or "get address of first element of array" so maybe we do want to have deleted overloads for them.

While there seems to be little reason for proxy to reject function pointers, this restriction was bypassed in this change. Some unit test cases were added accordingly.