bootstrap:table: sparse hashtable: keep table half-full at most
bootstrap:table: sparse hashtable: keep table half-full at most

This doesn't cost a lot in memory, as only the bitmap gets bigger, but
it's sensibly faster. We also gain by avoiding the division. Lowering
the limit further doesn't appear to help much.

nanocritical/n,nanocritical/n,nanocritical/n,nanocritical/n