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)`.
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