Panic-safety unsoundness in `Stack::pop`, `Fifo::pop_front` and `Value::replace_stable` (use-after-free / double-free)

Three removal and replacement paths destroy an initialized value before
committing the metadata change that removes it. Destruction runs `T::drop()`,
which is user code and may panic. If it does, the commit is skipped and the
container still claims ownership of the already-destroyed value, so its own
destructor drops that value a second time.

`Stack::pop` drops the top value before reducing `next_ofs`. `Stack`'s
destructor pops in a loop, so an unchanged `next_ofs` re-drops the same value.

`Fifo::pop_front` (via `pop_front_inner`) drops the front value before advancing
`read_pos`. An unchanged `read_pos` still points at the destroyed value.

`Value::replace_stable` drops the existing value before writing the replacement.
If the destructor panics the replacement is never written, but the backing
storage still holds the old value's metadata.

This is separate from RUSTSEC-2021-0033, which covered a clone panic on
insertion in `push_cloned` and was fixed in 0.6.1.

## Impact

A heap-owning value is freed twice, corrupting the allocator. Reachable from
safe Rust with any element type whose `Drop` can panic — no `unsafe` on the
caller's side.

* CWE-415 (Double Free)
* CWE-416 (Use-After-Free)

Confirmed under AddressSanitizer, which reports `attempting double-free` on all
three paths.

## Fix

Fixed in 0.8.2.