Preparatory work to facilitate utf8 transition

[Bodigrim]

The main work on utf8 transition is likely to be a pretty long-lived branch, but there are several preparatory changes, which improve development experience in general, and I'd like to merge them early, so that I do not spend time rebasing them.

When merging, please rebase, do not squash. Each commit is a separate, orthogonal change here, but raising 6 PRs seems an overkill.

[Bodigrim]

Discussion points:

1. One might argue that it's preferable to collect all integer conversions in a single module instead of scattering them around. I think there is not much overlap between modules at the moment to warrant such move, but I would not mind to do so, if others feel it as a better alternative.

2. Instead of repeating #ifdef ASSERT HasCallStack => #endif Bla -> Bla -> Bla everywhere, we can define in a separate module type HasCallStack' = #ifdef ASSERT HasCallStack #else () #endif and then just use HasCallStack' => Bla -> Bla -> Bla. Does it sound appealing to do so?

[Lysxia]

LGTM

[Lysxia]

Re: discussion points. I think the way you did things is fine. In particular for 2, refactoring into a type synonym unfortunately makes it visible to haddock.