implement chrono conversions for unusual duration types

implement chrono conversions for unusual duration types

Summary:
Implement conversions between std::chrono::duration and POSIX-style time
structures even when neither the numerator nor the denominator of the duration
ratio are 1.

Both of these are done by first converting to an intermediate type where the
numerator is 1, and then using the conversion routines for that case.

Reviewed By: yfeldblum

Differential Revision: D6366647

fbshipit-source-id: 8f9495fb4101cac6d8b4cf0353a679107007b298