Timings is a simple data structure that collects times of labeled
Start/Stop events describing timed phases, which later can be written
to a file.

Adjacent phases with common label prefix are automatically collected
in a group together with the accumulated phase time.

Timing data can be appended to a file in benchmark data format
using the new -bench flag:

$ go build -gcflags="-bench=/dev/stdout" -o /dev/null go/types
commit: devel +8847c6b Mon Aug 15 17:51:53 2016 -0700
goos: darwin
goarch: amd64
BenchmarkCompile:go/types:fe:init              1       663292 ns/op      0.07 %
BenchmarkCompile:go/types:fe:loadsys           1      1337371 ns/op      0.14 %
BenchmarkCompile:go/types:fe:parse             1     47008869 ns/op      4.91 %    10824 lines    230254 lines/s
BenchmarkCompile:go/types:fe:typecheck:top1    1      2843343 ns/op      0.30 %
BenchmarkCompile:go/types:fe:typecheck:top2    1       447457 ns/op      0.05 %
BenchmarkCompile:go/types:fe:typecheck:func    1     15119595 ns/op      1.58 %      427 funcs     28241 funcs/s
BenchmarkCompile:go/types:fe:capturevars       1        56314 ns/op      0.01 %
BenchmarkCompile:go/types:fe:inlining          1      9805767 ns/op      1.02 %
BenchmarkCompile:go/types:fe:escapes           1     53598646 ns/op      5.60 %
BenchmarkCompile:go/types:fe:xclosures         1       199302 ns/op      0.02 %
BenchmarkCompile:go/types:fe:subtotal          1    131079956 ns/op     13.70 %
BenchmarkCompile:go/types:be:compilefuncs      1    692009428 ns/op     72.33 %      427 funcs       617 funcs/s
BenchmarkCompile:go/types:be:externaldcls      1        54591 ns/op      0.01 %
BenchmarkCompile:go/types:be:dumpobj           1    133478173 ns/op     13.95 %
BenchmarkCompile:go/types:be:subtotal          1    825542192 ns/op     86.29 %
BenchmarkCompile:go/types:unaccounted          1       106101 ns/op      0.01 %
BenchmarkCompile:go/types:total                1    956728249 ns/op    100.00 %

For #16169.

Change-Id: I93265fe0cb08e47cd413608d0824c5dd35ba7899
Reviewed-on: https://go-review.googlesource.com/24462Reviewed-by: Matthew Dempsky <mdempsky@google.com>