alloc: reduce page header further, go down to 64k minimum.

This means we can't have more than 2^25 elements per page; that's
a maximum small page size of about 2^24 (with >8 objects per small page
we move to large pages), meaning a poolsize max of 4G.

We have a tighter limit at the moment anyway, but we should remove it
once we fix this.  In particular count all-zero and all-one words in
the used field (that's what we care about: full or empty) would give us
another factor of 64 (we only care about larger pool sizes on 64-bit
platforms).

We can also restore the larger number of pages and greater inter-page
spacing once we implement the alternative tiny allocator.