Implement more operators

[erikdubbelboer]

Add support for the following:

  $nor
  $not
  $in
  $nin
  $exists
  $elemMatch
  field = [array]
  field = null

[erikdubbelboer]

Closed in favor of separate pull requests for each change:

#8 Also fuzz test the JSONB code
#10 Make basicOperatorMap private
#11 Always return nil for empty values
#12 Add some different types of columns and data to our test setup
#13 Implement $nor
#14 Implement $not
#15 Plain comparison only works with primitives
#16 Support comparing to null
#17 Implement $exists
#18 Implement $nin
#19 Implement $elemMatch