Add some traits

[thewilsonator]

cc @ibuclaw

[dlang-bot]

Thanks for your pull request and interest in making D better, @thewilsonator! 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 annotated coverage diff directly on GitHub with CodeCov's browser extension
• 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 fetch digger
dub run digger -- build "master + druntime#2412"

[PetarKirov]

If we're going to add those traits to druntime, can we move them completely, by aliasing them from phobos?

[thewilsonator]

They're in core.internal.traits so no, not unless we move traits out of core.internal.

[wilzbach]

They're in core.internal.traits so no, not unless we move traits out of core.internal.

Why not?
We can do everything with .internal modules - even use public symbols there. And we have done so in the past too.