Misbehaving `HandleLike` implementation can lead to memory safety violation

Unsafe code in ObjectPool has time-of-check to time-of-use (TOCTOU) bug that
can eventually lead to a memory safety violation. ObjectPool and HandlePool
implicitly assumes that HandleLike trait methods are pure, i.e., they always
return the same value. However, this assumption is unsound since HandleLike
is a safe, public trait that allows a custom implementation.