Header error tests

[rvagg]

Also pulls in #74 and #75

Ref: ipld/js-car#28

Also included in this is an explicit check for 0aa16776657273696f6e02 (<length>{version:2}) as per ipld/specs#248 (comment) so it can be used as a CARv2 escape - if we end up using this string as a modifier for CARv2 we can at least get proper errors out of this if it encounters a newer format.

the primary difference with js-car is that go-car requires a non-empty roots array whereas js-car is fine with empty roots array, hence the test fixture differences

[mvdan]

Hm, why not merge #75 first? otherwise the review gets tricky :)

[masih]

I'm not entirely sure on the order of PRs that are open. If this pulls other PRs in, would it make sense to close the other ones?

Left a bunch of questions.
LGTM 🍻

[rvagg]

okie dokie, this is much simpler to review now, just the relevant changes and some .gitignore action