Re: Introducing --gc:arc

[cblake]

It appears as if this poor behavior began sometime between nim-0.19.2 and 
nim-0.19.4, but only for `--gc:markAndSweep` (which was in my nim.cfg -- 
oops!). The default GC works fine after all, albeit with a very noisy 
min50(max()). Sorry about the somewhat misleading report. There are probably 
glitches in many of the non-default gc's. `gc:arc` is still much faster with 
much better worst case times and uses half the memory of the old default.