Per #17973, RWMutex has some scaling issues. One option is to fix RWMutex, but for maps with high read:write ratios we can often do better with a carefully-managed atomic.Value anyway. The standard library contains many such maps, especially in the reflect package.

The actual details of such a map are a bit subtle to implement correctly and efficiently. We should provide a Map implementation in the standard library. Since the implementation may need to use both sync and sync/atomic and the former depends on the latter, it should either go in sync or in a new, separate sync subpackage.

(Presumably this should be targeted to 1.9 due to the 1.8 freeze, but I'd like to add a draft API to x/sync in the meantime.)