Due to the backward compatibility we won't be going to remove support
for `time::Duration` in 0.3, and the initial 0.3.0 release won't have
proper `std::time::Duration` support (haven't finalized the logics).
However we will reserve proper names and signatures for the upcoming
`std::time::Duration` support---the "older" duration type will be
referred as "signed" in the names.
- Added a `chrono::prelude` module. This does not have the (old)
`Duration` type reexported, so the documentation has now correctly
replaced all occurrences of `chrono::Duration`. The existing
`chrono::Duration` reexport itself remains for the compatibility.
- Avoided using a plain `Duration` type in the signature, to avoid
any ambiguity.
- Renamed `checked_{add,sub}` to `checked_{add,sub}_signed`.
- Subtraction operator between two instants has been removed and
replaced with `signed_duration_since`. This follows the naming
chosen by `std::time::SystemTime` etc., and the version for newer
`std::time::Duration` will be named to `duration_since`.