More smart variable.

[ilya-klyuchnikov]

Now variable are represented as names. The downside is that when we generate names for rebuildings memory is used a lot.

A variable is one from the following:

• Name
• Gen(subexpression, context)

As a benefit it will allow to test more interesting whistles - we will have a finite number of "true" free variables in configurations. - We may treat them as constants.