It can download single block raw leaves files over car.

I first meant it to be used for unixfs correctness checking, however 
later I've implemented it for range requests.

This make it fail at both, so let's keep correctness, I'll implement 
range requests later.

This fixes bug introduced in 4707f58.

This is was completely wrong (does not load the digest and incorrectly truncate the data incorrectly.)

```
$ benchstat /mnt/ramdisk/{old,new}
name   old time/op    new time/op    delta
PB-12     730ns ± 5%     255ns ± 2%   -65.04%  (p=0.000 n=9+10)

name   old alloc/op   new alloc/op   delta
PB-12      224B ± 0%        0B       -100.00%  (p=0.000 n=10+10)

name   old allocs/op  new allocs/op  delta
PB-12      4.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)
```

This needs:
- Use boxo for types and helpers
- Tests

Node is the easy to use API, don't confuse peoples with aliasable CIDs.

This used to be buggy because defer would be called in the loop.

We stopped checking roots, this only provides slightly better error messages when getting unknown car versions.
Remove dependency on cbor for this very small value.

This would make the code tricky to read and I'm not sure it would help in a significant way.