Find a way to properly free Isl context

[gdeest]

The runIsl function was previously implemented by bracketing an IO action with isl_ctx_alloc() and isl_ctx_free(). However, it seems that the freeingn action is called earlier than previously envisioned, which causes weird failures when manipulating sets associated to the context. The isl_ctx_free() call has thus been removed, which results a memory leak.