`bytes`: _to_smallvec to avoid allocation.

*On 2022-05-28 @recmo wrote in [`d2363fc`](https://github.com/recmo/uint/commit/d2363fc59b20c9ad1c827634b3003825dd02e2c6) “blocked”:*

_to_smallvec to avoid allocation.

```rust
ptr::{addr_of, addr_of_mut},
slice,
};
use std::borrow::Cow;

// OPT: *_to_smallvec to avoid allocation.

impl Uint {
/// The size of this integer type in bytes. Note that some bits may be
/// forced zero if BITS is not cleanly divisible by eight.
pub const BYTES: usize = (BITS + 7) / 8;

```
*From [`src/bytes.rs:15`](https://github.com/recmo/uint/blob/d2363fc59b20c9ad1c827634b3003825dd02e2c6/src/bytes.rs#L15)*