Summary:
Add folly::to() conversions to convert between std::chrono::duration or
std::chrono::time_point types and struct timespec or struct timeval types.

To conform to the behavior of the existing arithmetic-to-arithmetic
conversions, this code performs proper overflow checking and throws a
`ConversionError` on overflow.  This unfortunately does make the code rather
complicated compared to a non-checking implementation.

Conversions between some unusual duration types is not implemented yet, and
will fail at compile time if someone tries to use it.  This happens for
durations where neither the numerator nor the denominator of the ratio is 1.
For instance, 7/13ths of a second.

Reviewed By: yfeldblum

Differential Revision: D6356700

fbshipit-source-id: 9dce8ab8f32d8c18089f32c7176a8abf3c3f11f7