Extend PartiQL equality to graphs

[vgapeyev]

PartiQL equality, as defined in Section 7.1.1. of 2019 spec, defines how to compare any two values of any two types, in particular defining deep equality for collection values.

It now has to be extended to graphs as well.

The complication, however, is that graph isomorphism is a well-known hard problem. (Note that ids of nodes or edges cannot be used as "anchors" in this comparison, since they are not part of the data model -- two graphs with distinct ids for everything could still be isomorphic.)

This could be an opportunity to consider whether the current universal + deep equality of PartiQL should be somehow limited, and perhaps apply that limitation to comparing graphs. (See #12)