labeling MVar and STM data structures

[Kazu Yamamoto (=?iso-2022-jp?B?GyRCOzNLXE9CSScbKEI=?=)]

Hi Ben,

> This is generally in the direction I proposed in #21877; I agree that it
> would be quite useful. I don't think that we necessarily want to extend
> all MVars with labels as this would impose a fixed memory cost on all
> users. However, I think the approach that I describe in the ticket,
> introducing an out-of-band map for labelling heap objects, would be
> a reasonable way forward.

I support this.
Thanks.

--Kazu