Use std::size instead of sizeof(a) / sizeof(a[0]) pattern throughout the codebase.

[Ivorforce]

Identified this in a discussion.

In short, std::size is the canonical way to determine the number of elements in a C-style array starting from C++17. Using it looks slightly cleaner than the sizeof(a) / sizeof(a[0]) pattern, and is less error prone to regressions.
I took the opportunity to add constexpr to a few of the affected variables (where possible).

The change does have one downside, which is that #include <iterator> must be added in two spots. This is unlikely to observably affect compile times.

[Ivorforce]

FYI I got the compiler error fixed, but I'm waiting to rebase on top of #102396 when it's merged, to modify the sizeof it's adding :)

[Calinou]

Code looks good to me.

[Repiteo]

Thanks!