Re: Introducing --gc:arc

[cblake]

Well, changing to inherit from `CatchableError` did not alone fix things 
(though that is clearly a thing I should have done long ago, and I think 
separating bugs from unusual control flow is a Good Thing). I did catch these 
when compiled with `-d:danger/-d:release` for the past 2.5 years, though. I 
will try to work on a proper minimum reproducing example/bug report tomorrow. 
Been busy.