* Add HashJoinSegment, a virtual segment for joins.
An initial step towards #8728. This patch adds enough functionality to implement a joining
cursor on top of a normal datasource. It does not include enough to actually do a query. For
that, future patches will need to wire this low-level functionality into the query language.
* Fixups.
* Fix missing format argument.
* Various tests and minor improvements.
* Changes.
* Remove or add tests for unused stuff.
* Fix up package locations.