A possible solution to removing driver dependency from dialect

[aacanakin]

Currently, almost any structs, especially dialect depends on driver string. However, it seems it doesn't have to be that way.

Query struct would keep the placeholders in named statements. Only SQL() function should have driver dependency. Moreover, SQL() function should convert the named placeholders into native placeholders.

[aacanakin]

Named placeholder would not be necessary. Just putting "?" will be enough. For postgres, SQL(driver string) function should replace the "?" into binding indexed $%d. It looks like a ugly hack but is solves the current problem.