Indexes don't take advantage of performant structure

The common form of [Index](https://github.com/ipld/go-car/blob/3c99491a50b5a4adfbb187b4dba07bb6b37fbb7d/v2/index/index.go#L57) introduced in Car V2 is to lay out the CIDs in the car in a sorted table that can be quickly walked through with a binary search.

We should attempt to optimize not fully [copying the index into memory](https://github.com/ipld/go-car/blob/master/v2/index/indexsorted.go#L72) when loading a car, and just doing spot reads from the index. This can for large cars avoid a copy of hundreds of megs per individual block read.