`top` generates a query that is invalid for MSSQL

The `(top!)` query generates an SQL statement like `select ... from ... limit N`, but MSSQL does not accept that. There the result set can be reduced like `select top N from ...`.

Afaik, there is no way to change that in a custom 'backend', is it?

I don't know if adjusting the SQL generated from the expression is the best way to go, or allowing a library user to add new sqlosure expressions, so that I could write my own `mssql-top`.