Introduce BooleanVar, IntVar, LongVar, DoubleVar?

[fge]

A Var is a rather heavy beast:

• since it allows for nulls, instead of a light ArrayDeque, we need a LinkedList;
• the initial value is itself a generic class.

Starting with 1.1.0, Var will not allow for nulls (there will be a NullableVar for that). But the primitive case will not be solved.

Another motivation is issue #17.