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

[cblake]

The same problem happens on nim-1.2.0 (well, nim-devel git hash 
ed44e524b055b985b0941227a61f84dd8fdfcb37). So, this is a long-lived, maybe 
since the beginning behavior of `gc:arc` (but we should still have a memory 
overuse regression test). Probably time to look at generated C.