Weak reference facility in Emacs.

[Zhu Zihao]

Weak reference is good idea to deal with cycle reference. Though Emacs has GC,
but weak reference is also useful when I want to use RAII pattern in Emacs (via
finalizer).

hash table has option to make reference in it become weak. But it's not
so convenience if user just want a weak reference of *ONE* lisp object.

Would you interest in some lisp library to wrap a weak table to provide weak
referenece utils. Or it's better to implement it in alloc.c?