'constants' (values created at parse-time) will no longer be GC-ed, this
is more of a temporary fix so I can continue implementing GC and leave
the constant problem to be solved later.
plot can now garbage collect temporary values crated during function
call (for passing as arguments), for (fibo 31) this improves maximum
memory usage by 33% (4038826 plot_value(s) vs 2692574)