fmt.Scan:

fmt.Scan:
- reimplement integer scanning to handle renamed basic integer types
- start implementation of Fscanf and Scanf; not yet ready for general use.

This intermediate CL is a useful checkpoint. A similar change needs to be
done for float and complex, but it seemed worth getting the approach
reviewed before making those changes.

R=rsc
CC=golang-dev
https://golang.org/cl/1418041