Provide literal operators for ndn::time::duration types
They should be equivalent to the operators in
std::literals::chrono_literals, except that the literal suffix identifiers must start with an underscore character, as required by the C++ standard:
- Literal suffix identifiers that do not start with an underscore are reserved for future standardization.
Unfortunately we cannot use the standard
chrono_literals directly because they would return an
std::chrono::duration, which is incompatible with