Checked integrals is in reviewable state

[Andrei Alexandrescu via Digitalmars-d]

Code: https://github.com/dlang/phobos/pull/4613

Docs: 
http://dtest.thecybershadow.net/artifact/website-e0b2b4609f1a8e4010ccbe9492ab4af3e3fdd2fb-ab9e77dd6d78136b1f0172e60f7b4f43/web/phobos-prerelease/std_experimental_checkedint.html

New policies implement NaN and saturation with relative ease. I think it 
shouldn't be too hard to add bounds checks in a future iteration. 
Anyhow, reviews would be very welcome.


Andrei