This is based on (cribbed almost directly from) code written by the
inimitable @StephanTLavavej on one of our mailing lists.

This is a nice generic version of the approach used in
JsonUtils::EnumMapper and CodepointWidthDetector: a static array of
key-value pairs that we binary-search at runtime (or at compile time, as
the case may be.)

Keys are not required to be sorted, as we're taking advantage of
constexpr std::sort (VS 16.6+) to get the compiler to do it for us. How
cool is that?

static_map presents an operator[] or at much like
std::map/std::unordered_map does.

I've added some tests, but they're practically fully-solveable at compile
time so they pretty much act like `VERIFY_IS_TRUE(true)`.

…_map