Implement Ord trait for Timespec

[cpeterso]

Note that Timespec does not perform any range checking and this change does not add any!

This implementation of Ord assumes that pre-epoch Timespecs have negative sec and positive nsec fields. Linux's and Darwin's struct timespec functions handle pre-epoch timestamps with this "two steps back, half step forward" representation, though I cannot find any documentation that actually puts this in writing. This means that (say) -1.2 seconds is represented by Timespec { sec: -2_i64, nsec: 800_000_000_i32 }, not something like Timespec { sec: -1_i64, nsec: -200_000_000_i32 } or Timespec { sec: -1_i64, nsec: 200_000_000_i32 }.

If we wish to codify this implementation detail, we could make Timespec nsec unsigned and add range checks asserting nsec <= 999_999_999_u32. btw, struct Tm also lacks range checking and uses signed integers for fields that can't be negative.