Redesign TimeVal and TimeSpec

[kamalmarhubi]

(TimeSpec is being added in #276)

These types aim to provide a nice Rust struct abstraction over the libc types. However, they could be revisited. Some issues I noticed:

• all the ops trait impls lose range on the values by calling num_microseconds() (num_nanoseconds()) for implementation convenience. This reduces the usable range of TimeVal to +/- ~300k years (probably ok) and TimeSpec to +/- ~300 years (not really ok).
• std::Duration has been stable since 1.3.0, and we should provide interconversions, and probably deprecate all the hours() and such methods in favour of converting from a Duration. (This brings up version support policy Decide on a rust version support policy #238.)

[Susurrus]

Closing this in favor of #516 which has some more discussion in it.