add method `to_sql` and small refactoring

[ermolaev]

$mini_sql.to_sql('select ?',  1)

[ermolaev]

Hello @SamSaffron, what do you think of this PR?

[SamSaffron]

I think it looks good, we are adding one call frame?

[ermolaev]

yes, to_sql is public method, we already have to_sql in builder

this PR unifies the public API and shortens the code, for example

SQL = MINI_SQL.build(<<~SQL).to_sql(state: LineItem::STATE::IN_DELIVERY)
  SELECT * from line_items WHERE order_id = :order_id AND state = :state
SQL

SQL = MINI_SQL.to_sql(<<~SQL, LineItem::STATE::IN_DELIVERY)
  SELECT * from line_items WHERE order_id = :order_id AND state = ?
SQL

MINI_SQL.prepared.query(SQL, order_id: order.id)