consider new join syntax leveraging key definitions

[agentm]

Joins in TutorialD are currently a bit clunky because one must rename attributes to be equal. Instead, we propose new syntactic sugar to generate the renames from declared foreign keys:

Proposed syntax:

rvA join rvB using foreign key rvA_rvB_fk

This syntax should use the existing attribute names, if possible, otherwise the expression could create arbitrarily-named attributes, which is bad for reproducibility.