Added a new function to the right-hand side of an unknown - demand.

The new constraint system functional form definition is val system : v -> ((v -> d) -> (v -> d -> unit) -> (v -> unit) -> d) m, where the added (v -> unit) represents demand.
Thread spawning now uses demandl (Function fd, c); inplace of ignore (getl (Function fd, c)).

TD3 implements demand as demand_thr and adds a new hashmap weak_dep.
TD3 with weak dependencies can be run in two "modes" - eager and lazy.

Eager.
Can be toggled with --enable solvers.td3.wk_deps.eager

1. If demand is called the demanded unknown y is added to the hashmap weak_dep and solved immediately.
2. If at the end of the iteration any unknown in weak_dep or in vs is unstable, they are solved again until all unknowns in both are stable.

Lazy:
Default behavior currently.
Same as eager, but without solving immediately when calling demand.

Part of the bachelor's thesis "Weak Dependencies for Side-Effecting Constraint Systems".