Quiets most const warnings in the fractal heap code

[derobins]

The biggest problem here is the 'generic operation' scheme that combines a const void pointer to arbitrary data + a callback that can sometimes modify said arbitrary data, thus undermining the const-ness in certain cases. Since nobody is going to rearchitect this scheme anytime soon, I've at least quieted the warnings that we know are not a problem since the "sometimes const" pointers won't be modified when they point to const data given the existing code.

In case anyone is wondering, making the operation callbacks simply take a non-const pointer is a bit like pulling on the loose yarn in a sweater. I started going down this road, but I think this is a more minimalist and cleaner solution for now.

I left the file pointer const warnings in the cache client code alone. That's a can of worms I'm unwilling to touch at this time and I also don't want to just glibly turn off the cast warnings.

[gnuoyd]

This is quite a wordy way to de-const a pointer.

Other C projects de-const pointers without making the compiler warn by using a macro or function that casts through uintptr_t. Something like this should do the trick,

static inline void *
deconst(const void *p)
{
    return (void *)(uintptr_t)p;
}

The H5HF__man_op_real() call would turn to H5HF__man_op_real(hdr, id, H5HF__op_write, deconst(obj), H5HF_OP_MODIFY).