Use abstract function name for ECall

[kevinresol]

For example "IF" on mysql is "IIF" in mssql, and there is no such function at all in sqlite

We might need some AST to hold the function and concretize it in a formatter.

[benmerckx]

I wondered about it too since they're so specific to the database used. An alternative would be:

import tink.sql.expr.Functions.*; // count/max/min etc
import tink.sql.expr.Functions.MySql.*;
import tink.sql.expr.Functions.Sqlite.*;
// etc