Pursuant to a pghackers discussion back around 11-Jul-00, get rid of aset.c's

not-very-good handling of mid-size allocation requests.  Do everything via
either the "small" case (chunk size rounded up to power of 2) or the "large"
case (pass it straight off to malloc()).  Increase the number of freelists
a little to set the breakpoint between these behaviors at 8K.