fmt: use public io.RuneScanner interface for ScanState reader

fmt: use public io.RuneScanner interface for ScanState reader

All io.Reader that are passed to newScanState in all the standard
library tests that implement io.RuneReader also implement io.RuneScanner.

Do not check on each call ScanState's UnreadRune that the used RuneReader
also implements the UnreadRune method by using a private interface.
Instead require the used Reader to implement the public RuneScanner
interface.

The extra implementation logic for UnreadRune is removed from ScanState.
Instead the readRune wrapper is extended to implement UnreadRune for the
RuneScanner interface. If the Reader passed to newScanstate does not
implement RuneScanner the readRune wrapper is used to implement the
missing functionality.

Note that a RuneReader that does not implement RuneScanner will also
be wrapped by runeRead which was not the case before.
Performance with the readRune wrapper is better than without before.

Add benchmark to compare performance with and without using the
readRune wrapper.

name                             old time/op  new time/op  delta
ScanInts-2                        704µs ± 0%   615µs ± 1%  -12.73%  (p=0.000 n=20+20)
ScanRecursiveInt-2               82.6ms ± 0%  51.4ms ± 0%  -37.71%  (p=0.000 n=20+20)
ScanRecursiveIntReaderWrapper-2  85.1ms ± 0%  52.4ms ± 0%  -38.36%  (p=0.000 n=20+20)

Change-Id: I8c6e85db9b87a8171caab12f020b6e256b498e81
Reviewed-on: https://go-review.googlesource.com/19895
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>