- sign to determine if a value is < 0, == 0, > 0
- abs to compute absolute value
- Rat.IsInt to test if a rational number is representable as an integer
R=rsc
CC=golang-dev
https://golang.org/cl/1761042
* Allow an exponent part. This is necessary for exp/eval.
* Fix a bug for input that had no numbers after the decimal.
* In Int.SetString, allow a leading + sign.
* In Int.SetString, error if the input is "-" with no number.
* In nat.scan, normalize the resulting nat.
R=gri
CC=golang-dev
https://golang.org/cl/1733045