Flaw in generativity allows out-of-bounds access

Affected versions of this crate did not properly implement the generativity,
because the invariant lifetimes were not necessarily dropped.

This allows an attacker to mix up two arenas, using indices created from one
arena with another one. This might lead to an out-of-bounds read or write
access into the memory reserved for the arena.

The flaw was corrected by implementing generativity correctly in version 0.4.0.