Add benchmarks for map assignment with int32/int64/string key

Benchmark results on darwin/amd64

name                  old time/op  new time/op  delta
MapAssignInt32_255-8  24.7ns ± 3%  17.4ns ± 2%  -29.75%  (p=0.000 n=10+10)
MapAssignInt32_64k-8  45.5ns ± 4%  37.6ns ± 4%  -17.18%  (p=0.000 n=10+10)
MapAssignInt64_255-8  26.0ns ± 3%  17.9ns ± 4%  -31.03%  (p=0.000 n=10+10)
MapAssignInt64_64k-8  46.9ns ± 5%  38.7ns ± 2%  -17.53%  (p=0.000 n=9+10)
MapAssignStr_255-8    47.8ns ± 3%  24.8ns ± 4%  -48.01%  (p=0.000 n=10+10)
MapAssignStr_64k-8    83.0ns ± 3%  51.9ns ± 3%  -37.45%  (p=0.000 n=10+9)

name                     old time/op    new time/op    delta
BinaryTree17-8              3.11s ±19%     2.78s ± 3%    ~     (p=0.095 n=5+5)
Fannkuch11-8                3.26s ± 1%     3.21s ± 2%    ~     (p=0.056 n=5+5)
FmtFprintfEmpty-8          50.3ns ± 1%    50.8ns ± 2%    ~     (p=0.246 n=5+5)
FmtFprintfString-8         82.7ns ± 4%    80.1ns ± 5%    ~     (p=0.238 n=5+5)
FmtFprintfInt-8            82.6ns ± 2%    81.9ns ± 3%    ~     (p=0.508 n=5+5)
FmtFprintfIntInt-8          124ns ± 4%     121ns ± 3%    ~     (p=0.111 n=5+5)
FmtFprintfPrefixedInt-8     158ns ± 6%     160ns ± 2%    ~     (p=0.341 n=5+5)
FmtFprintfFloat-8           249ns ± 2%     245ns ± 2%    ~     (p=0.095 n=5+5)
FmtManyArgs-8               513ns ± 2%     519ns ± 3%    ~     (p=0.151 n=5+5)
GobDecode-8                7.48ms ±12%    7.11ms ± 2%    ~     (p=0.222 n=5+5)
GobEncode-8                6.25ms ± 1%    6.03ms ± 2%  -3.56%  (p=0.008 n=5+5)
Gzip-8                      252ms ± 4%     252ms ± 4%    ~     (p=1.000 n=5+5)
Gunzip-8                   38.4ms ± 3%    38.6ms ± 2%    ~     (p=0.690 n=5+5)
HTTPClientServer-8         76.9µs ±41%    66.4µs ± 6%    ~     (p=0.310 n=5+5)
JSONEncode-8               16.5ms ± 3%    16.7ms ± 3%    ~     (p=0.421 n=5+5)
JSONDecode-8               54.6ms ± 1%    54.3ms ± 2%    ~     (p=0.548 n=5+5)
Mandelbrot200-8            4.45ms ± 3%    4.47ms ± 1%    ~     (p=0.841 n=5+5)
GoParse-8                  3.43ms ± 1%    3.32ms ± 2%  -3.28%  (p=0.008 n=5+5)
RegexpMatchEasy0_32-8      88.2ns ± 3%    89.4ns ± 2%    ~     (p=0.333 n=5+5)
RegexpMatchEasy0_1K-8       205ns ± 1%     206ns ± 1%    ~     (p=0.905 n=5+5)
RegexpMatchEasy1_32-8      85.1ns ± 1%    85.5ns ± 5%    ~     (p=0.690 n=5+5)
RegexpMatchEasy1_1K-8       365ns ± 1%     371ns ± 9%    ~     (p=1.000 n=5+5)
RegexpMatchMedium_32-8      129ns ± 2%     128ns ± 3%    ~     (p=0.730 n=5+5)
RegexpMatchMedium_1K-8     39.8µs ± 0%    39.7µs ± 4%    ~     (p=0.730 n=4+5)
RegexpMatchHard_32-8       1.99µs ± 3%    2.05µs ±16%    ~     (p=0.794 n=5+5)
RegexpMatchHard_1K-8       59.3µs ± 1%    60.3µs ± 7%    ~     (p=1.000 n=5+5)
Revcomp-8                   1.36s ±63%     0.52s ± 5%    ~     (p=0.095 n=5+5)
Template-8                 62.6ms ±14%    60.5ms ± 5%    ~     (p=0.690 n=5+5)
TimeParse-8                 330ns ± 2%     324ns ± 2%    ~     (p=0.087 n=5+5)
TimeFormat-8                350ns ± 3%     340ns ± 1%  -2.86%  (p=0.008 n=5+5)

name                     old speed      new speed      delta
GobDecode-8               103MB/s ±11%   108MB/s ± 2%    ~     (p=0.222 n=5+5)
GobEncode-8               123MB/s ± 1%   127MB/s ± 2%  +3.71%  (p=0.008 n=5+5)
Gzip-8                   77.1MB/s ± 4%  76.9MB/s ± 3%    ~     (p=1.000 n=5+5)
Gunzip-8                  505MB/s ± 3%   503MB/s ± 2%    ~     (p=0.690 n=5+5)
JSONEncode-8              118MB/s ± 3%   116MB/s ± 3%    ~     (p=0.421 n=5+5)
JSONDecode-8             35.5MB/s ± 1%  35.8MB/s ± 2%    ~     (p=0.397 n=5+5)
GoParse-8                16.9MB/s ± 1%  17.4MB/s ± 2%  +3.45%  (p=0.008 n=5+5)
RegexpMatchEasy0_32-8     363MB/s ± 3%   358MB/s ± 2%    ~     (p=0.421 n=5+5)
RegexpMatchEasy0_1K-8    4.98GB/s ± 1%  4.97GB/s ± 1%    ~     (p=0.548 n=5+5)
RegexpMatchEasy1_32-8     376MB/s ± 1%   375MB/s ± 5%    ~     (p=0.690 n=5+5)
RegexpMatchEasy1_1K-8    2.80GB/s ± 1%  2.76GB/s ± 9%    ~     (p=0.841 n=5+5)
RegexpMatchMedium_32-8   7.73MB/s ± 1%  7.76MB/s ± 3%    ~     (p=0.730 n=5+5)
RegexpMatchMedium_1K-8   25.8MB/s ± 0%  25.8MB/s ± 4%    ~     (p=0.651 n=4+5)
RegexpMatchHard_32-8     16.1MB/s ± 3%  15.7MB/s ±14%    ~     (p=0.794 n=5+5)
RegexpMatchHard_1K-8     17.3MB/s ± 1%  17.0MB/s ± 7%    ~     (p=0.984 n=5+5)
Revcomp-8                 273MB/s ±83%   488MB/s ± 5%    ~     (p=0.095 n=5+5)
Template-8               31.1MB/s ±13%  32.1MB/s ± 5%    ~     (p=0.690 n=5+5)

Updates #19495

Change-Id: I116e9a2a4594769318b22d736464de8a98499909
Reviewed-on: https://go-review.googlesource.com/38091Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>