Add spec file for filesystem-less repository manipulation.
Use a GTOID directly.
Dot accessor.
Add GTOID initializers for data + object type.
const-ify arguments.
`stream` isn't freed.
Refactor the streaming write call to be more "general".

If someone gets an epiphany on how to do that without passing the git_odb_stream object to the block or copying the data…
Add a convenience call to create tree entries and their data at the same time.
Convenience call to create commits.
Spec for the easy commit creation.