Re: NvP: s.add('x') 100M times

[cblake]

I don't see how your linked algo explains deltas across gcs if that `3 div 2` 
growth happens for all of them. The memory `gc:arc` uses here seems more like 
the sum of all prior allocations, not "up to 1.5x what's needed". Actually, 
`gc:arc` uses 1.25x more mem than `gc:none` (312MB) in a test I just tried.