enum aliases / unknown_read support

[jtanx]

There are times when enum values are renamed but we still want to be able to read old names. Some built-in way to define aliases or otherwise providing an unknown_read option for enums would be nice to make this simpler to do

[stephenberry]

Generally it's good for the JSON to match the underlying C++. In C++ different enum names cannot have identical values. I understand the desire for this feature, but it would also need to be implemented in a manner that wouldn't reduce runtime performance for typical enum usage.

I'll keep this issue alive for more discussion and I'll think about it more before implementing anything.

[jtanx]

Supposing some kind of unknown_read option was presented, wouldn't that keep the perf the same as it's all compile time/constexpr conditional on that method existing? E.g. if the alias/unknown_method option were missing then there is no penalty, if it is defined then you're asking for more complex handling