fixes IN and #587

[sviezypan]

This PR fixes sql 's IN which was not well defined.
You can call IN with any sequence of values like

select(fkProductId).from(productPrices).where(price in List(10, 20, 74))

or you use IN with subquery

val subquery = select(age).from(users).where(age > 18)
select(fName).from(users).where(age in subquery)

Also fixes #587 so the following query

select(fName, lName, orderDate).from(customers.join(orders).on(fkCustomerId == customerId))

will yield error message (there should be 3 equal signs ===)

Use === instead of == when comparing two Exprs