Add more SQL helpers

[esentio]

Add more helpers for SQL functions.

[esentio]

Since addition of Exp.Raw<T> in #62, there is a simple way how to add chunks of raw SQL to the query. Especially to SELECT clause where it was not possible to use raw SQL before and we had to rely on SQL helpers.

The point of SQL helpers is not to blindly port every SQL function. Function (or SQL expression, ...) represented by the helper should be very common to justify porting it to managed code. Or ideally, helper method should provide an additional value, like for example DateTime.SameDay() method.

Quite certainly, there will be added new useful SQL helpers in the future. But I'm closing this for now.