Add std.atomic support

[rymrg]

Add std.atomic struct as a better interface to core.atomic.

@atilaneves Please CR and let me know what's missing here.

[dlang-bot]

Thanks for your pull request and interest in making D better, @rymrg! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#10864"

[atilaneves]

This LGTM but I'd like another pair of eyes looking at this. Anyone you can think of?

[thewilsonator]

Enforce Allman style
grep -nrE '(if|for|foreach|foreach_reverse|while|unittest|switch|else|version) .*{$' $(find etc std -name '*.d'); test $? -eq 1
std/atomic.d:13:struct Atomic(T) if (__traits(isIntegral, T) || isPointer!T) {
std/atomic.d:76:	T opUnary(string op)() shared if (op == `++`) {
std/atomic.d:80:	T opUnary(string op)() shared if (op == `--`) {
std/atomic.d:84:	auto ref opUnary(string op)() shared if (op == `*`) {
std/atomic.d:123:@safe unittest {
std/atomic.d:146:@safe unittest {
std/atomic.d:152:@system unittest {
std/atomic.d:163:		while (arr[1].load!(MemoryOrder.acq) != 1) {
std/atomic.d:176:@safe unittest {
std/atomic.d:183:@safe unittest {
std/atomic.d:191:@safe unittest {
std/atomic.d:199:unittest {

braces on newlines

[thewilsonator]

Enforce space after cast(...)
grep -nrE '[^"]cast\([^)]*?\)[[:alnum:]]' $(find etc std -name '*.d') ; test $? -eq 1
std/atomic.d:110:	rlx = cast(int)core.atomic.MemoryOrder.raw,
std/atomic.d:115:	acq = cast(int)core.atomic.MemoryOrder.acq,
std/atomic.d:120:	rel = cast(int)core.atomic.MemoryOrder.rel,
std/atomic.d:125:	acq_rel = cast(int)core.atomic.MemoryOrder.acq_rel,
std/atomic.d:130:	seq = cast(int)core.atomic.MemoryOrder.seq,

[thewilsonator]

Not quite sure what it wants with respect to documentation and unittests. I guess you need Params: and Returns: sections?

std/atomic.d(13:8)[warn]: Public declaration 'Atomic' has no documented example.
std/atomic.d(13:15)[warn]: Template parameters T isn't documented in the `Params` section.
std/atomic.d(13:18)[warn]: If constraints should have the same indentation as the function
std/atomic.d(35:21)[warn]: Template parameters mo isn't documented in the `Params` section.
std/atomic.d(41:25)[warn]: Template parameters mo isn't documented in the `Params` section.
std/atomic.d(41:49)[warn]: Parameter newVal isn't documented in the `Params` section.
std/atomic.d(50:2)[warn]: A public function needs to contain a `Returns` section.
std/atomic.d(50:21)[warn]: Template parameters mo isn't documented in the `Params` section.
std/atomic.d(50:45)[warn]: Parameter mod isn't documented in the `Params` section.
std/atomic.d(56:2)[warn]: A public function needs to contain a `Returns` section.
std/atomic.d(56:21)[warn]: Template parameters mo isn't documented in the `Params` section.
std/atomic.d(56:45)[warn]: Parameter mod isn't documented in the `Params` section.
std/atomic.d(62:2)[warn]: A public function needs to contain a `Returns` section.
std/atomic.d(62:25)[warn]: Template parameters mo isn't documented in the `Params` section.
std/atomic.d(62:49)[warn]: Parameter desired isn't documented in the `Params` section.
std/atomic.d(68:2)[warn]: A public function needs to contain a `Returns` section.
std/atomic.d(68:23)[warn]: Template parameters mo isn't documented in the `Params` section.
std/atomic.d(68:57)[warn]: Template parameters fmo isn't documented in the `Params` section.
std/atomic.d(68:82)[warn]: Parameter oldVal isn't documented in the `Params` section.
std/atomic.d(68:92)[warn]: Parameter newVal isn't documented in the `Params` section.
std/atomic.d(74:27)[warn]: Template parameters mo isn't documented in the `Params` section.
std/atomic.d(74:61)[warn]: Template parameters fmo isn't documented in the `Params` section.
std/atomic.d(74:86)[warn]: Parameter oldVal isn't documented in the `Params` section.
std/atomic.d(75:6)[warn]: Parameter newVal isn't documented in the `Params` section.
std/atomic.d(81:2)[warn]: A public function needs to contain a `Returns` section.
std/atomic.d(81:22)[warn]: Template parameters op isn't documented in the `Params` section.
std/atomic.d(81:28)[warn]: Parameter rhs isn't documented in the `Params` section.
std/atomic.d(87:2)[warn]: A public function needs to contain a `Returns` section.
std/atomic.d(87:19)[warn]: Template parameters op isn't documented in the `Params` section.
std/atomic.d(92:4)[warn]: Public declaration 'opUnary' is undocumented.
std/atomic.d(97:11)[warn]: Public declaration 'opUnary' is undocumented.
std/atomic.d(104:6)[warn]: Public declaration 'MemoryOrder' is undocumented.
std/atomic.d(222:1)[warn]: A unittest should be annotated with at least @safe or @system