Solve possible calc value returned by evaluation of var id

[lucafavatella]

The cache semantics (as specified in the Feb 2013 paper) is wrong as it does not guarantee that rule (9) (i.e. eval) returns a value (or list of missing dimensions) - it could return a calc! (... that would blow up the rule that called this rule)

The problem is that in rule (9) there is a separate final call to beta.find, that assumes that the chain just written by rule (10) and, ultimately, by rule (11) was not garbage collected (this assumption is wrong for concurrency among threads in the interaction with the cache).

The solution is modifying rule (9):

• Remove beta.find, i.e. return the result of rule (10) eval1; and
• Do not pass the full context k but k restricted to the set of known dimensions.

This solution implies that when the chain of a variable is being computed by rule (10), i.e. eval1, the chain will be built only using the subset of the context that is known.

[lucafavatella]

Rebased on current master.