ReadVersion can be more efficient

* if carv1 header is of size != 10 then it is most-likely a v1
* If the carv1 header begins with a non-empty roots list then it is most likely a v1.
* If the carv1 header begins with an empty roots list, then we skip it (fixed length) and read the version.
* If we did that, we'd always read a fixed length, so we could just take a []byte.

We wouldn't support carv1-looking headers whose version is not 1, but the carv1 spec requires the version to always equal 1.

Context: https://github.com/ipld/go-car/pull/100#pullrequestreview-691564461