renamed `n<foo>s` to `num_<foo>s` and changed their semantics.

`Duration` now has a `to_tuple` method which corresponds to
the original `(dur.ndays(), dur.nseconds(), dur.nnanoseconds())`.
new `num_*s()` methods in `Duration` always return the integral
number of specified units in the duration.

cf. rust-lang/rust#15934