Add infix operators for relational algebra

This PR includes yet moar Unicode; this time, primarily to support [relational algebra](http://en.wikipedia.org/wiki/Relational_algebra).

1. Allows all Unicode characters in the [geometric shapes block](http://en.wikipedia.org/wiki/Geometric_Shapes) as valid characters.

2. Defines joins (⨝ ⟕ ⟖ ⟗) and antijoin (▷) as valid infix operators. (Note that ▷ is semantically a shape, but I couldn't find a separate antijoin character.)
If joins are infix operators, then they don't also need to go in `is_cat_id_start`.  Normally we put them in one place or the other.