core.lifetime: Add copyEmplace() to simulate copy-construction

[kinke]

With a function signature analogous to moveEmplace() - source ref first, then target ref. Allowing non-mutable target types T is required to invoke the correct copy constructor, see the tests.

std.variant has a similar implementation (but insufficient wrt. copy constructors), and there might be more potential usage sites in
druntime/Phobos/user code.

Destructing a partially copied static array has been adopted from copy-constructing a new dynamic array:

druntime/src/core/internal/array/construction.d

Lines 49 to 71 in b7391cd

	size_t i;
	try
	{
	for (i = 0; i < to.length; i++)
	{
	auto elem = cast(Unqual!T*)&to[i];
	// Copy construction is defined as bit copy followed by postblit.
	memcpy(elem, &from[i], element_size);
	postblitRecurse(*elem);
	}
	}
	catch (Exception o)
	{
	/* Destroy, in reverse order, what we've constructed so far
	*/
	while (i--)
	{
	auto elem = cast(Unqual!T*)&to[i];
	destroy(*elem);
	}
	throw o;
	}

[dlang-bot]

Thanks for your pull request and interest in making D better, @kinke! 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 + druntime#3239"

[thewilsonator]

Looks fine to me. cc @RazvanN7

[RazvanN7]

Why is this marked as @system?

[kinke]

I guess it was because of overwriting target, without destructing it first, so it's dangerous.

[kinke]

Oh and because it accepts non-mutable target types T too.

[RazvanN7]

Makes sense, thanks!