First observed in est31/serde-big-array#19,
there's no performant way to serialize fixed-size byte arrays with
serde/postcard.

Add a `FixedSizeByteArray` type that can be used to serialize fixed-size
byte arrays faster than all other available methods. This type only
works with postcard. A more general solution would need to be
implemented in serde itself.

This works around serde-rs/serde#2680.

```
serialize16/own                 [3.8846 ns 3.8909 ns 3.8988 ns]
serialize16/bytes               [9.9192 ns 9.9503 ns 9.9907 ns]
serialize16/byte_array          [4.1374 ns 4.1461 ns 4.1564 ns]
serialize16/big_array           [68.245 ns 68.344 ns 68.454 ns]
serialize16/fixed_size          [9.9966 ns 10.030 ns 10.076 ns]
serialize16/variable_size       [15.696 ns 15.795 ns 15.898 ns]

serialize32/own                 [4.1744 ns 4.1857 ns 4.2014 ns]
serialize32/bytes               [9.6590 ns 9.6825 ns 9.7151 ns]
serialize32/byte_array          [4.5584 ns 4.5663 ns 4.5747 ns]
serialize32/big_array           [135.38 ns 135.78 ns 136.34 ns]
serialize32/fixed_size          [13.989 ns 14.034 ns 14.091 ns]
serialize32/variable_size       [21.223 ns 21.270 ns 21.328 ns]

deserialize16/own               [5.9018 ns 5.9143 ns 5.9272 ns]
deserialize16/bytes             [21.198 ns 21.278 ns 21.376 ns]
deserialize16/byte_array        [6.8560 ns 6.8719 ns 6.8934 ns]
deserialize16/big_array         [19.027 ns 19.098 ns 19.187 ns]
deserialize16/fixed_size        [7.9197 ns 7.9471 ns 7.9817 ns]
deserialize16/variable_size     [36.804 ns 36.871 ns 36.946 ns]

deserialize32/own               [8.8860 ns 8.8989 ns 8.9139 ns]
deserialize32/bytes             [21.241 ns 21.271 ns 21.321 ns]
deserialize32/byte_array        [11.441 ns 11.459 ns 11.481 ns]
deserialize32/big_array         [31.553 ns 31.618 ns 31.697 ns]
deserialize32/fixed_size        [17.334 ns 17.360 ns 17.386 ns]
deserialize32/variable_size     [59.705 ns 59.815 ns 59.949 ns]
```

- `own` is the new `FixedSizeByteArray`, no length prefix.
- `bytes` is `serde_bytes::Bytes`, it has a length prefix.
- `byte_array` is `serde_byte_array::ByteArray`, it has a length prefix.
- `big_array` is `serde_big_array::Array`, no length prefix.
- `fixed_size` is `[u8; _]`, no length prefix.
- `variable_size` is `[u8]`, it has a length prefix.

They're used less often in real code.