Release large PSLP presolve buffers early

[hlinsen]

• PSLP now frees its transpose matrix after presolve, removing approximately 12.1 GB on mcf_2500_100_500.
• Expose an API to free the reduced problem free_reduced_problem
• After cuOpt copies the reduced problem, it releases PSLP’s duplicate matrix, bounds, row sides, and objective arrays, removing another approximately 8.3 GB—about 20.4 GB total.

Presolver* p = new_presolver(...);
run_presolver(p);
free_presolver_reduced_problem(p);  // Optional early release after copying reduced LP
postsolve(p, ...);
free_presolver(p);        // Final free

[dance858]

Awesome! Thank you for this very nice contribution. I made some small edits. Feel free to merge if you think my edit on line 762 of presolver.c looks good.

Many thanks!