Ensuring hash tuples are properly maxaligned

I've been poking around in the PHJ code trying to identify the reason
why there are still so many buildfarm failures. I've not nailed it
down yet, but one thing I did notice is that there's an entirely
undocumented assumption that offsetof(HashMemoryChunkData, data) is
maxalign'ed. If it isn't, the allocation code will give back non-
maxaligned pointers, which will appear to work as long as you only
test on alignment-lax processors.

Now, the existing definition of the struct seems safe on all
architectures we support, but it would not take much to break it.
I think we ought to do what we did recently in the memory-context
code: insert an explicit padding calculation and a static assertion
that it's right. Hence, the attached proposed patch (in which
I also failed to resist the temptation to make the two code paths
in dense_alloc() look more alike). Any objections?

regards, tom lane