GitLab

Use the codec dedicated to CAR index sorted when marshalling and
unmarshalling `indexSorted`. Note, the code depends on a specific commit
of `go-multicodec` `master` branch. This needs to be replaced with a tag
once a release is made on the go-multicodec side later.

Unexport index APIs that should not be exposed publicly. Remove
`Builder` now that it is not needed anywhere. Move `insertionIndex` into
`blockstore` package since that's the only place it is used.

Introduce an index constructor that takes multicodec code and
instantiates an index.

Fix ignored errors in `indexsorted.go` during marshalling/unmarshlling.

Rename index constructor functions to use consistent terminology; i.e.
`new` instead if `mk`.

Remove redundant TODOs in code.

Relates to:
- https://github.com/multiformats/go-multicodec/pull/46

Address review comments

* Rename constructor of index by codec to a simpler name and update
docs.

* Use multicodec.Code as constant instead of wrappint unit64 every time.