Add computation_info object.
Mark the call as a created reg.
Actually don't allocate index-vars.
Record more created regs.
Small clean up.
Record force counts.
Default the force_count to 0.
Update docs.
Record which computation we are calling.
Update weak_ref::reset() and add weak_ref::is_null().
Fix assert in evaluate.C
Change call_comp to weak_ref.
Correctly force called computations.

Only force them one time.
Remove unspecified_bool_type hack in favor of explicit operator bool.
Add reg_heap::force_computation( ) and reg_heap::unforce_computation( ).
Allow calling unforce_computation( ) on a weak_ref.
Reorder member initializers to match constructor.
Add initial framework to unforce when computations are destroyed.
Try to unforce used/forced results before destroying a computation.
Add source token.