get_unsat_core for Optimize

[poscat0x04]

Since there is support for getting unsat core from the optimizer in the C API/z3-sys, it would be nice to add it to the high level API as well.

[waywardmonkeys]

I'll look at doing this in the next day or two if no one else submits it first.

[poscat0x04]

Also, the unsat core generated by z3 will only contain assertions tracked using Z3_optimize_assert_and_track, which exists in the C API but for some reason isn't available in z3-sys

[waywardmonkeys]

I've just added the binding to z3-sys for Z3_optimize_assert_and_track. Still working to find the time to do the rest.

[poscat0x04]

no pressure :)