Checkedint ready for one more round of reviews

[Andrei Alexandrescu via Digitalmars-d]

Just made one more pass through it addressing concerns and adding a new 
policy (Throw). Reviews welcome. Let's get this through Scylla and 
Charybdis!

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

Dox: 
http://dtest.thecybershadow.net/artifact/website-c1933a4611fdfd4f996f85d2f37a669f34d58d58-2020d60f92d3b96ef9c94dc8b38c7723/web/library-prerelease/std/experimental/checkedint.html

Docs: 
http://dtest.thecybershadow.net/artifact/website-c1933a4611fdfd4f996f85d2f37a669f34d58d58-2020d60f92d3b96ef9c94dc8b38c7723/web/phobos-prerelease/std_experimental_checkedint.html


Andrei