Add infallable ck_rhs_reset_preallocated

[michael-grunder]

Currently, when you want to flush a ck_rhs_t you must call ck_rhs_reset[_size] which can fail under low memory conditions (when trying to allocate the backing map with hs->m->malloc).

If it does fail, the only choice you have is to unwind the map in-place, which while possible is less than ideal.

This commit gives us an infallable pathway, allowing the user to preallocate one or more backing maps ahead of time for later use.

Two new functions are introduced:

/* Returns the internally rounded up size required for an underlyinig
   ck_rhs_map for the number of elements requested */
size_t ck_rhs_map_size(ck_rhs_t *, unsigned long);

/* Similar to ck_rhs_reset_size except the caller provides the
   underlying map allocation, making it infallible */
void ck_rhs_reset_preallocated(ck_rhs_t *, unsigned long, void *);

[michael-grunder]

I tried to make as minimal a change to the code as possible to lower the cognitive load for merging.

The only other way of making the reset infallable could work would be using globals inside of the attached allocator but that's brittle if libck ever changes.

static bool inRhsReset;

void *rhs_malloc(size_t size) {
    if (inRhsReset) {
        return getPreallocatedMap(size);
    }
    
    return myRealMalloc(size);
}

[sbahra]

Thank you!