Panic-safety unsoundness in `truncate_back`, `truncate_front`, `clear`, and `extend_from_slice` (use-after-free / double-free)

Several methods in `circular-buffer` drop elements before updating the buffer's start/end metadata. If an element's `Drop` panics during the drop, the metadata update is skipped, so the buffer still treats the already-dropped elements as live. When the buffer is later dropped (or used after the panic is caught), those elements are visited again — a use-after-free / double-free reachable from safe Rust.

## Impact

- **CWE-415 (Double Free):** the same allocation is freed twice.
- **CWE-416 (Use-After-Free):** a freed allocation is accessed during a repeated `Drop`.

Reachable entirely from safe Rust via `catch_unwind` with element types whose `Drop` can panic.

## Fix

Fixed in `circular-buffer` 2.0.1 by adjusting the buffer's start/end metadata before the elements are dropped.