fmt: cleanup and optimize doPrintf for simple formats

fmt: cleanup and optimize doPrintf for simple formats

Make a fast path for format strings that do not use
precision or width specifications or argument indices.

Only check and enforce the restriction to not pad left with zeros
in code paths that change either f.minus or f.zero.

Consolidate the if chains at the end of the main doPrintf loop
into a switch statement. Move error printing into extra
functions to reduce size of this switch statement.

name                             old time/op  new time/op  delta
SprintfPadding-2                  234ns ± 1%   233ns ± 1%   -0.54%  (p=0.010 n=19+19)
SprintfEmpty-2                   37.0ns ± 3%  39.1ns ±14%     ~     (p=0.501 n=17+20)
SprintfString-2                   112ns ± 1%   101ns ± 1%   -9.21%  (p=0.000 n=19+20)
SprintfTruncateString-2           139ns ± 1%   139ns ± 0%   +0.57%  (p=0.000 n=19+19)
SprintfQuoteString-2              402ns ± 0%   392ns ± 0%   -2.35%  (p=0.000 n=19+20)
SprintfInt-2                      114ns ± 1%   102ns ± 2%  -10.92%  (p=0.000 n=20+20)
SprintfIntInt-2                   177ns ± 2%   155ns ± 2%  -12.67%  (p=0.000 n=18+18)
SprintfPrefixedInt-2              260ns ± 3%   249ns ± 3%   -4.55%  (p=0.000 n=20+20)
SprintfFloat-2                    190ns ± 1%   178ns ± 2%   -6.54%  (p=0.000 n=20+20)
SprintfComplex-2                  533ns ± 1%   517ns ± 3%   -2.95%  (p=0.000 n=20+20)
SprintfBoolean-2                  102ns ± 1%    93ns ± 2%   -9.30%  (p=0.000 n=20+20)
SprintfHexString-2                176ns ± 0%   168ns ± 2%   -4.49%  (p=0.000 n=16+19)
SprintfHexBytes-2                 181ns ± 1%   174ns ± 2%   -4.27%  (p=0.000 n=20+20)
SprintfBytes-2                    326ns ± 1%   311ns ± 1%   -4.51%  (p=0.000 n=20+20)
ManyArgs-2                        540ns ± 2%   497ns ± 1%   -8.08%  (p=0.000 n=18+16)
FprintInt-2                       150ns ± 0%   149ns ± 0%   -0.33%  (p=0.000 n=20+18)
FprintfBytes-2                    185ns ± 0%   165ns ± 0%  -10.98%  (p=0.000 n=20+18)
FprintIntNoAlloc-2                113ns ± 0%   112ns ± 0%   -0.88%  (p=0.000 n=20+20)

Change-Id: I9ada8faa1f46aa67ea116a94ab3f4ad3e405c8fe
Reviewed-on: https://go-review.googlesource.com/20919
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>