Serialization of std::chrono between GNU GCC and LLVM Clang not compatible

[marcosbento]

It is known that GNU GCC's standard library implementation currently uses std::chrono::nanoseconds ticks, while LLVM Clang's standard library implementation uses std::chrono::microseconds ticks. This mismatch is disregarded by the current Cereal implementation (https://github.com/USCiLab/cereal/blob/master/include/cereal/types/chrono.hpp), which serializes ticks regardless of units, and makes an object serialized in GCC incompatible with the one serialized in LLVM.

Could we consider updating Cereal's implementation to always convert to/from std::chrono::nanoseconds, and thus ensure compatibility?