Allocating StablePtrs leads to GC slowdown even after they're freed

If we allocate and then free a lot of StablePtrs the GC performance will be degrated for the rest of the execution.

I have attached a program that performs a GC-heavy task (foldr'ing a long list of Ints) before and after allocating and then freeing a million StablePtrs. After the StablePtrs are freed the task takes more than twice as long.

The reason for this is that stable_ptr_table in Stable.c is never resized, and is looped over for every GC pause.