const / constexpr review

[tap]

At least in the parts of the code where I've been involved, I think we have done a fair job of using const consistently and intentionally for methods and the like. Maybe less so for variables. We should do a sweep across the code base and make sure we're sane everywhere on this as it's only really effective if we are consistent and not casting it away.

Also for constants we have a mix of macros and consts, many of which could be replaced with constexpr. It's unclear to me just how marginal the Microsoft compiler is about constexpr so that needs to factor into any changes in this department.