v2: use a byte array for the characteristics bitfield

Right now, characteristics is [as follows](https://pkg.go.dev/github.com/ipld/go-car/v2@v2.0.2#Characteristics):

```
type Characteristics struct {
Hi uint64
Lo uint64
}
```

And, as per the code, it encodes and decodes as little-endian.

If Go had a uint128 type, I'd definitely agree with this approach. For better or worse it doesn't, so using `encoding/binary.LittleEndian` doesn't save us complexity - we still have to choose between Hi and Lo.

I think it would be easier to use `[16]byte`, as proposed by @rvagg here: https://github.com/ipld/go-car/pull/239#discussion_r711861809

Then the logic should also become fairly straightforward to follow. For example, a "get bit" or "set bit" method on Characteristics would use `pos/8` to select an array index, and `pos%8` to select the bit within its byte.