sync/atomic: add typed atomic values

sync/atomic: add typed atomic values

These implementations will inline to the lower-level primitives,
but they hide the underlying values so that all accesses are
forced to use the atomic APIs. They also allow the use of shorter
names (methods instead of functions) at call sites, making code
more readable.

Pointer[T] also avoids conversions using unsafe.Pointer at call sites.

Discussed on #47141.
See also https://research.swtch.com/gomm for background.

Fixes #50860.

Change-Id: I0b178ee0c7747fa8985f8e48cd7b01063feb7dcc
Reviewed-on: https://go-review.googlesource.com/c/go/+/381317
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>