feat(zql): Implement OR

[arv]

This supersedes #52

OR is implemented by branching the graph when an OR expression is encountered. Each OR branch is executed "in parallel" and then merged together. The merging is done using concat and then distinct.

For example if we have:

SELECT * FROM table WHERE (a=1 OR b=1) AND (a=2 OR b=2)

We get something like:

  graph TD;
      Root-->id1;
      Root-->id2;
      id1(a=1) --> c1;
      id2(b=1) --> c1;
      c1(Concat) --> d1;
      d1(Distinct) --> id3;
      d1 --> id4;
      id3(a=2) --> c2;
      id4(b=2) --> c2;
      c2(Concat) --> d2;
      d2(Distinct)

[arv]

I have builder changes but that can be reviewed in isolation.

[tantaman]

Will review this tonight. Very cool :)