Easier debugging support

[slightlyoff]

It should be possible at every point to determine what constraints determined the value for a particular Variable once a solution is found. The API for this might be something like:

c.extend(c.Variable.prototype, {
   get constraininedBy() {
       return [ /*c.LinearExpression*/ ... ];
   },
   get unconstrainedBy() {
       return [ /*c.LinearExpression*/ ... ];
   },
});

[infty-]

+1