consider new join syntax leveraging key definitions

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.