Prefer constexpr over const

[skyline75489]

This isn't all of them. If you like how it goes, I'll file another PR to address the rest.

[YakovL]

@skyline75489 could you explain what this is supposed to improve/make less fragile? I guess, some explanation (may be a link to a language guideline?) will help @derceg to decide whether this should be merged

[derceg]

Thanks for the pull request, I think this change is worth making.

As there have been a number of updates since this pull request was created, it won't merge as-is (e.g. because some of the constants have been removed). Could you rebase your changes on top of master?

In terms of a second pull request, that makes sense. For consistency, at least, I think it's best to have all of the existing instances updated, either in one pull request, or two.

[hsupu]

seems constexpr const is better?