Replace FieldProxy with @property ref functions

[pbackus]

This simplifies the code and removes the need for a separate
version (StdDdoc) block.

[dlang-bot]

Thanks for your pull request and interest in making D better, @pbackus! 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#8259"

[UplinkCoder]

Nice!

[CyberShadow]

LGTM. Unfortunately I can't guess why I didn't do it this way in 2011, ref properties did seem to be working at that point.

[pbackus]

Oops, I forgot the function attributes the first time around (and the test suite didn't catch it—could use better coverage here). Should be good to go now.

[maxhaton]

Nice

[pbackus]

Auto-tester is failing on FreeBSD with this message:

core.exception.AssertError@std/process.d(1645): Assertion failure

The test in question is here:

phobos/std/process.d

Lines 1639 to 1647 in 232ca53

	// last resort, try lsof (not available on all Posix)
	TestScript lsof = "lsof -p$$";
	auto lsofRes = execute(lsof.path, null);
	if (lsofRes.status == 0)
	{
	assert(!lsofRes.output.canFind(path));
	assert(execute(lsof.path, null, Config.inheritFDs).output.canFind(path));
	return;
	}

I don't see how this test could possibly be affected by this PR, so I'm going to assume for now it's a flaky test and force-push to re-run.