Add `EXISTS` unary operator, fix #216

[RomainMazB]

First draft to fix #216 , the next step would be to handle the wildcard to constraint a SELECT * without any further columns.

[RomainMazB]

This PR is ok for review, but rely on #219 , so it should be merged before this one.

There will be some conflict inside the tests, I renamed the methods name so you can just manually merge the tests in the good order.

[tyt2y3]

As discussed in another PR, I think we don't need to introduce a new UnarySelect variant, instead we can extend the existing SubQuery variant to include one more enum probably SubQueryOper which could itself be Exists or None

[ikrivosheev]

Close PR, continue: https://github.com/SeaQL/sea-query/pull/379