00000004-CLOSED-add_constexpr_to_everything.md

# Add `constexpr` to everything

## Explanation

There is a lot of code in Crucible that can reasonably be made `constexpr`. Without putting any
additional work into actually enabling _more_ code to be `constexpr`, add it wherever it will
currently fit without causing compilation to break.

## Task

Practically, for each namespace which has some inline definitions (so an `.inl` file), go through
every inline function (and variable, if applicable) and add `constexpr` unless it won't work. If it
doesn't compile straight away and there's no obvious localized way to make it work, leave it for
the moment.