code review 136710045: sync/atomic: add Value

sync/atomic: add Value
A Value provides an atomic load and store of a consistently typed value.
It's intended to be used with copy-on-write idiom (see the example).

Performance:
BenchmarkValueRead	50000000	        21.7 ns/op
BenchmarkValueRead-2	200000000	         8.63 ns/op
BenchmarkValueRead-4	300000000	         4.33 ns/op

[dvyukov, 2014-09-17 02:52:26]

Hello golang-codereviews@googlegroups.com,

I'd like you to review this change to
https://dvyukov%40google.com@code.google.com/p/go/

[dvyukov, 2014-09-17 02:53:53]

*** Submitted as https://code.google.com/p/go/source/detail?r=b279bb7f7c3b ***

sync/atomic: add Value
A Value provides an atomic load and store of a consistently typed value.
It's intended to be used with copy-on-write idiom (see the example).

Performance:
BenchmarkValueRead	50000000	        21.7 ns/op
BenchmarkValueRead-2	200000000	         8.63 ns/op
BenchmarkValueRead-4	300000000	         4.33 ns/op

TBR=rsc
R=golang-codereviews
CC=golang-codereviews
https://codereview.appspot.com/136710045

[dvyukov, 2014-09-17 02:53:59]

This is a resend of already LGTMed https://codereview.appspot.com/120140044/