GitLab

Move index generation functionality to root, because they read/write CAR
files. This leaves the `index` package to only contain the index
implementation itself.

Move `index.Attach` to root while at it following the same logic. Add
TODOs to the implementation of Attach to make it more robust. As is it
_could_ result in corrupt v2 files if offset includes padding since v2
header is not updated in this function.

Add tests to generate index and move existing tests to correspond to the
go file containing generation functionality.