fix build errors caused by purely namespaced enums and druation api changes

[nanne007]

1. Switch to purely namespaced enums rust-lang/rust#18973 made a change to enum namespacing, which causes a build error.
2. num::abs method no longer exists, use SignedInt.abs instead.
3. Zero trait implementation for Duration is removed. Instead, use Duration.zero() directly.

[fantix]

That's neat! Thanks for the fix!