safemem is unmaintained

The latest crates.io release was in 2019. The repository has been archived by the author.

## Migration

- `safemem::copy_over(slice, src_idx, dest_idx, len);` can be replaced with `slice.copy_within(src_idx..src_idx+len, dest_idx);` as of `rust 1.37.0`.
- `safemem::write_bytes(slice, byte);` can be replaced with `slice.fill(byte);` as of `rust 1.50.0`
- `safemem::prepend(slice, vec);` can be replaced with

```rust
let old_len = vec.len();
vec.extend_from_slice(slice);
vec.rotate_left(old_len);
```

as of `rust 1.26.0`