unused import of core::slice in block-buffer

```
warning: unused import: `core::slice`
  --> /usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/block-buffer-0.7.3/src/lib.rs:11:5
   |
11 | use core::slice;
   |     ^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default
```