Since we require C++11, most of these consts can be constexpr.
Two `static const ld` remain non-compile-time-evaluable because
they depend on the runtime `log` function. One `static const cld`
remains non-compile-time because `std::complex<T>` doesn't become
constexpr until C++14.
Apple Xcode has started giving `-Wdeprecated-declarations` warnings
for `sprintf`, and suggesting that people migrate to `snprintf` instead.
This is silly, but the warnings are spam and need to be silenced
somehow. Migrating to `snprintf` and/or `hr::format` is the path of
least resistance.
