BigArrays: reuse pages more agressively

[jpountz]

Current implementation starts using paged arrays when the size of the array is
greater than the size of a page. This means that if an array has a length of
${page_size}+1, it will use two pages, wasting about half of the memory.

I think this behavior should be symetric when the size is less than the page
size but higher than half of the page size. This means that we would start
reusing memory at ${page_size}/2 instead of ${page_size}.

[nik9000]

Makes sense to me though it might be worth running performance numbers on it. It might be that you should kick in even earlier. I don't have any ideas on how to measure the performance of such a change though.

[jpountz]

I thought about making it kick in earlier but then I would be afraid of wasting too much memory. However, since we are wasting 50% of space in the worst case anyway, I thought it would be useful to take advantage of it to reuse pages more agressively.

[nik9000]

That's pretty sound logic to me.