Cabal flag to control auto-labelling of threads

[jasagredo]

Having this cabal flag allows the user to, on-demand, auto-label the async threads, pointing to the place where the async task is spawned. The idea is that the user then can go to the code/dependency and add a meaningful label to that thread.

Using CPP makes it transparent for users that don't want to opt-in to this behavior.

My idea is for this PR to be merged after #163. The first commit in this branch is the same as in #163.

[simonmar]

I'm open to the idea of labelling the internal threads - we now have two PRs for that (#162 and #163, I slightly prefer #162). But I'm not keen on adding auto-labelling functionality via cabal flags and CPP. Could you do this using a wrapper package async-labelled instead? It could use the same module name Control.Concurrent.Async so client code doesn't have to change.

[phadej]

Could you do this using a wrapper package async-labelled instead

That won't be ergonomic. If there's some other pakcage using async, you would need go and change it's .cabal file to use async-labelled; so broadly speaking some code would need to be changed.

Compared to adding --constraint="async +debug-auto-label" to cabal build command.