Make use of the fat arrow in a couple of places
Add Relation#inRepository

This method makes a clone of the receiving relation,
but using the given repository (which may have a different
DB connection, and later, a different identity map).
This will be used by the sandbox.
Clean up specs

- Share definitions of test record classes
- Move matchers into their own file
- Move fixture data into its own file
Move transaction method from Record to Relation
Generate correct SQL for nested selections
In CompositeTuple#toString, use constructor.name

(not .displayName)
Refactor sandbox and middleware