- better number scanning algorithm
- fixed a couple of bugs related to base interpretation
- added scan benchmark
- added more test cases and made tests more precise
- introduced Int.scan method matching nat.scan
- refactored Int.Scan; now uses int.scan
- refactored Int.SetString; now uses int.scan

There is more potential, this was a fairly simple change.

gotest -test.bench="ScanPi" before/after (best of 3 runs):
big.BenchmarkScanPi	   1000	    2024900 ns/op
big.BenchmarkScanPi       10000      257540 ns/op

R=chickencha
CC=golang-dev, rsc
https://golang.org/cl/4527089