Adds check that, for example, a Scan method taking
a first argument of type fmt.ScanState has the correct
signature to satisfy fmt.Scanner.

Similarly, a ReadByte should return byte, os.Error.

These are important to check because various pieces
of code (fmt, gob, json, flate) do dynamic checks
for these methods, so code with incorrect signatures
would not be flagged at compile time.

These become even more important to check when
rune is introduced.

R=r
CC=golang-dev
https://golang.org/cl/5305044