Finalizer for Env

[yeesian]

similar to jump-dev/Gurobi.jl#24.

Currently KNITRO.jl doesn't call freeProblem in the finalizer; I haven't found a way to make it work. The relevant lines:

(lines 57-62 in src/KNITRO.jl)

   function KnitroProblem()
       prob = new(newcontext())
       # finalizer segfaults upon termination of the running script
       # finalizer(prob, freeProblem)
       prob
   end

and

(lines 67-70 in src/KNITRO.jl)

   function freeProblem(kp::KnitroProblem)
       freecontext(kp.env)
       #kp.env = C_NULL
   end

cc @IainNZ @mlubin

[IainNZ]

Can you add some printlns to figure out which line segfaults? I don't see why that would segfault unless you try to operate on the env after freeing it.