Once upon a time fmt did use bytes.Buffer for its buffer.
The buffer write methods still mimic the bytes.Buffer signatures.
The current code depends on manipulating the buffer []bytes array directly
which makes going back to bytes.Buffer by only changing the type of buffer
impossible. Since type buffer is not exported the methods can be simplified
to the needs of fmt. This saves space and avoids unnecessary overhead.

Use WriteString instead of Write for known inputs since
WriteString is faster than Write to append the same data.
This also saves space in the binary.

Remove the add method from Printer and depending on the data to be written
use WriteRune or WriteByte directly instead.

In total makes the go binary around 4 kilobyte smaller.

name                  old time/op  new time/op  delta
SprintfEmpty-2        24.1ns ± 3%  23.8ns ± 1%  -1.14%  (p=0.000 n=20+20)
SprintfString-2        114ns ± 2%   114ns ± 4%    ~     (p=0.558 n=20+19)
SprintfInt-2           116ns ± 9%   118ns ± 7%    ~     (p=0.086 n=20+20)
SprintfIntInt-2        195ns ± 6%   193ns ± 5%    ~     (p=0.345 n=20+19)
SprintfPrefixedInt-2   251ns ±16%   241ns ± 9%  -3.69%  (p=0.024 n=20+19)
SprintfFloat-2         203ns ± 4%   205ns ± 5%    ~     (p=0.153 n=20+20)
SprintfBoolean-2       101ns ± 7%    96ns ±11%  -5.23%  (p=0.005 n=19+20)
ManyArgs-2             651ns ± 7%   628ns ± 7%  -3.44%  (p=0.002 n=20+20)
FprintInt-2            164ns ± 2%   158ns ± 2%  -3.62%  (p=0.000 n=20+18)
FprintfBytes-2         215ns ± 1%   216ns ± 1%  +0.58%  (p=0.000 n=20+20)
FprintIntNoAlloc-2     115ns ± 0%   112ns ± 0%  -2.61%  (p=0.000 n=20+20)
ScanInts-2             700µs ± 0%   702µs ± 1%  +0.38%  (p=0.000 n=18+20)
ScanRecursiveInt-2    82.7ms ± 0%  82.7ms ± 0%    ~     (p=0.820 n=20+20)

Change-Id: I0409eb170b8a26d9f4eb271f6292e5d39faf2d8b
Reviewed-on: https://go-review.googlesource.com/19955Reviewed-by: Rob Pike <r@golang.org>