Static factory methods in Exp for conditions

[andrus]

Let's add static expression factory methods to the Exp interface to build conditions. They will supercede the current Exp.keyValue, Exp.simple. E.g.:

Exp.not()
Exp.equal()
Exp.less()
Exp.lessOrEqual()
Exp.in()
Exp.notIn()
... etc

Note that we'll need to work some more on the expresion builder API beyond this task to accommodate more complex expressions with functions, arithmetics, etc.