Add Fact concept

[fmbenhassine]

As of v3.x, a fact is represented by Map.Entry<String, Object>. From a design point of view, this is a poor representation for a key domain concept like "fact". This design has a couple of drawbacks:

• It is not type safe in regards to fact values
• It is not convenient to iterate over a set facts
• It is not easy/clean to enforce fact invariants (unique names, non null values, etc)

A Fact concept should be introduced to represent and encapsulate a named fact with its typed value:

public class Fact<T> {
	private final String name;
	private final T value;
}