Fix regression and overflow bug for rationals.
Merge pull request #1 from jbcrail/fix-rational-rounding

Fix regression and overflow bug for rationals.
Rename `Zero` variant to `NoSign`
Merge pull request #3 from japaric/rename-zero

Backport: Rename `Zero` variant to `NoSign`
Add a README for num
Merge pull request #4 from steveklabnik/master

Add a README for num