This commit changes RUBY_GC_HEAP_INIT_SIZE_{40,80,160,320,640}_SLOTS to
RUBY_GC_HEAP_{0,1,2,3,4}_INIT_SLOTS. This is easier to use because the
user does not need to determine the slot sizes (which can vary between
32 and 64 bit systems). They now just use the heap names
(`GC.stat_heap.keys`).