Allow filter with join

[myguidingstar]

currently only current entity's columns are allowed in filters
There should be :join-filters and :extra-join-filters to provide constraints about join's properties.

{[(:person/all {:filters      {:person/number [:> 0]}
                :join-filters {:person/pets {:pet/yob [:> 1999]}}})]
 [:person/name :person/yob]}

SELECT `person`.`name` AS `person/name`, `person`.`yob` AS `person/yob`
FROM `person`
WHERE
(`person`.`number` IN (SELECT `person_pet`.`person_number`
                      FROM `person_pet` JOIN `pet` ON `person_pet`.`pet_index` = `pet`.`index`
                      WHERE `pet`.`yob` > 1999))
AND
(`person`.`number` > 0)

should make use of #22

[myguidingstar]

should not separate join-filters from filters, instead allow join filters as a new kind of filters so they can be combined freely (with and, or)

[myguidingstar]

the above example becomes:

{[(:person/all {:filters {;; in-table filter
                          :person/number [:> 0]
                          ;; join filter
                          :person/pets {:pet/yob [:> 1999]}}})]
 [:person/name :person/yob]}

[myguidingstar]

• extract the function that generates the above sub query (the SELECT inside an IN)
  • should also have sql query and params separated
• modify the filter spec to include join filters
• modify the WHERE emitter to include params introduced by sub queries