rfl::Generic: add support for long (int64)

[bjia56]

While working on code that converts rfl::Generic to JSON and back, I found that reflect-cpp@v0.17.0 does not properly convert int64's through Generic. Namely, Generic is defined as a variant with int as one option, which causes the JSON parser to cast yyjson values to int, potentially truncating the number.

This change introduces long as a variant option under Generic. It is placed before int to prioritize converting numbers as the bigger variant in the JSON parser, as otherwise the parser will indiscriminately match all integers to int. However, this likely has the side effect where all ints are now treated as longs. As a corrective measure, to_int and to_long will fall back and static cast from the other type if the variant does not directly match. I was usure of the impact of upgrading the variant's int to long (instead of the proposed implementation of adding a new variant), but perhaps replacing int with long would be cleaner?

If an alternative implementation would be better, please let me know!

[liuzicheng1987]

@bjia56 , some of the tests fail, could you take a look?