Add a flag for avoiding various boot libs

[infinity0]

I'm experimenting with de-duplicating StrictPair/StrictMaybe out of the containers library and need this flag to break the dependency cycle strict -> binary -> containers -> strict.

[phadej]

Is this discussed with maintainers of containers (and GHC). Until then I'll leave this unmerged.

[infinity0]

Well, sjakobi had expressed a potential interest in having strict-containers be part of containers, and one stepping stone to that would be to have strict also as a dependency. A concrete discussion hasn't started yet, but it's easier to have one when I can open a PR that tests OK on CI. To do that it would be necessary to release these PRs, then one can add the necessary constraints: flags to cabal.project as part of a demo PR to containers.

[phadej]

I really want to be sure about that GHC is fine of containers adding strict as dependency and making strict (and these) boot libraries and potentially causing some dependency reversals. I want to see a discussion about this. Therefore I tag @chessai @emilypi, @bgramari @treeowl

I'm in support of pushing that all that code into "bundled with GHC" basket, but there have to be a broad agreement and clear plan.

Concretely: boot flag is no-go if strict becomes an actual boot library, the "commented-out" code have to go somewhere.

[infinity0]

Yes, there is further work to be done if it were actually a boot library. My point was that having this PR included in a release on Hackage, makes it easier to explore the possibilities because then public PRs don't need to point to private copies of strict or these.

The discussion is a little bit premature at the moment because I didn't explore the issue concretely yet, but I imagine turning Map, HashMap, etc into typeclasses containing basic constructors/destructors as methods, so that separate lazy vs strict types can implement those, similar to how Vector.Generic works already. Not sure if @sjakobi had any other thoughts. But I meant to take this slowly, didn't want to rush anyone into a discussion yet.

[phadej]

I'm not accepting experimental features in proper releases, master.

Make a sdist from a branch, upload it somewhere, use packages: https://.../these-999.9.tar.gz in cabal.project.