cmd/benchstat: import from rsc.io/benchstat

I copied the code from various dependencies in go-moremath
into a single 'internal/stats' package. That package is at the top
level of the repo because I expect to pull much of benchcmp
into an importable package.

For golang/go#14304.

Change-Id: Ie114839b2901f5060c202feb3ffc768bf43ce5da
Reviewed-on: https://go-review.googlesource.com/35503
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>

cmd/benchstat/README.md

@@ -0,0 +1,85 @@
+# Benchstat
+
+Benchstat computes and compares statistics about benchmarks.
+
+Usage:
+
+    benchstat [-delta-test name] [-geomean] [-html] old.txt [new.txt] [more.txt ...]
+
+Each input file should contain the concatenated output of a number of runs
+of ``go test -bench.'' For each different benchmark listed in an input file,
+benchstat computes the mean, minimum, and maximum run time, after removing
+outliers using the interquartile range rule.
+
+If invoked on a single input file, benchstat prints the per-benchmark
+statistics for that file.
+
+If invoked on a pair of input files, benchstat adds to the output a column
+showing the statistics from the second file and a column showing the percent
+change in mean from the first to the second file. Next to the percent
+change, benchstat shows the p-value and sample sizes from a test of the two
+distributions of benchmark times. Small p-values indicate that the two
+distributions are significantly different. If the test indicates that there
+was no significant change between the two benchmarks (defined as p > 0.05),
+benchstat displays a single ~ instead of the percent change.
+
+The -delta-test option controls which significance test is applied: utest
+(Mann-Whitney U-test), ttest (two-sample Welch t-test), or none. The default
+is the U-test, sometimes also referred to as the Wilcoxon rank sum test.
+
+If invoked on more than two input files, benchstat prints the per-benchmark
+statistics for all the files, showing one column of statistics for each
+file, with no column for percent change or statistical significance.
+
+The -html option causes benchstat to print the results as an HTML table.
+
+## Example
+
+Suppose we collect benchmark results from running ``go test -bench=Encode''
+five times before and after a particular change.
+
+The file old.txt contains:
+
+    BenchmarkGobEncode   	100	  13552735 ns/op	  56.63 MB/s
+    BenchmarkJSONEncode  	 50	  32395067 ns/op	  59.90 MB/s
+    BenchmarkGobEncode   	100	  13553943 ns/op	  56.63 MB/s
+    BenchmarkJSONEncode  	 50	  32334214 ns/op	  60.01 MB/s
+    BenchmarkGobEncode   	100	  13606356 ns/op	  56.41 MB/s
+    BenchmarkJSONEncode  	 50	  31992891 ns/op	  60.65 MB/s
+    BenchmarkGobEncode   	100	  13683198 ns/op	  56.09 MB/s
+    BenchmarkJSONEncode  	 50	  31735022 ns/op	  61.15 MB/s
+
+The file new.txt contains:
+
+    BenchmarkGobEncode   	 100	  11773189 ns/op	  65.19 MB/s
+    BenchmarkJSONEncode  	  50	  32036529 ns/op	  60.57 MB/s
+    BenchmarkGobEncode   	 100	  11942588 ns/op	  64.27 MB/s
+    BenchmarkJSONEncode  	  50	  32156552 ns/op	  60.34 MB/s
+    BenchmarkGobEncode   	 100	  11786159 ns/op	  65.12 MB/s
+    BenchmarkJSONEncode  	  50	  31288355 ns/op	  62.02 MB/s
+    BenchmarkGobEncode   	 100	  11628583 ns/op	  66.00 MB/s
+    BenchmarkJSONEncode  	  50	  31559706 ns/op	  61.49 MB/s
+    BenchmarkGobEncode   	 100	  11815924 ns/op	  64.96 MB/s
+    BenchmarkJSONEncode  	  50	  31765634 ns/op	  61.09 MB/s
+
+The order of the lines in the file does not matter, except that the output
+lists benchmarks in order of appearance.
+
+If run with just one input file, benchstat summarizes that file:
+
+    $ benchstat old.txt
+    name        time/op
+    GobEncode   13.6ms ± 1%
+    JSONEncode  32.1ms ± 1%
+    $
+
+If run with two input files, benchstat summarizes and compares:
+
+    $ benchstat old.txt new.txt
+    name        old time/op  new time/op  delta
+    GobEncode   13.6ms ± 1%  11.8ms ± 1%  -13.31% (p=0.016 n=4+5)
+    JSONEncode  32.1ms ± 1%  31.8ms ± 1%     ~    (p=0.286 n=4+5)
+    $
+
+Note that the JSONEncode result is reported as statistically insignificant
+instead of a -0.93% delta.