CVE-2021-29931: Double free in arenavec

Affected versions of this crate did not guard against potential panics that may happen from user-provided functions T::default() and T::drop().

Panic within T::default() leads to dropping uninitialized T, when it is invoked from common::Slice::<T, H>::new(). Panic within T::drop() leads to double drop of T, when it is invoked either from common::SliceVec::<T, H>::resize_with() or common::SliceVec::<T, H>::resize()

Either case causes memory corruption in the heap memory.