Add relational algebra unicode characters

[aaronsheldon]

Thank you @JeffBezanson, the unicode extensions you added in resolving #6929 are great. Could relational algebra operators be added as well?

• left
• right
• outer
• inner

I'm working on a tensor join package that will eventual use hash join/dictionaries for synchronized index iteration, and I would like to use the appropriate standard symbols to represent the joins.

[JeffBezanson]

I believe these were added in #8036