Add constexpr support to internal::sorted_map and internal::sorted_set

[wravery]

Fix #139

Unfortunately, constexpr support for std::vector seems to be lagging behind in GCC and AppleClang, so I was only able to do this by extracting some of the lookup logic an templatizing it on the container and iterator types in internal::sorted_map_equal_range and internal::sorted_map_lookup. That way it can apply the same logic to perform lookups against a std::array<std::pair<K, V>> as it would for an internal::sorted_map<K, V>.

The only place where I've actually adopted this is in the generated enum value maps, but this should make looking up an enum value O(log(n)) instead of O(n) at runtime. It also lets you perform compile time mapping between the strings and their enum values.