checkedint submission

[Andrei Alexandrescu via Digitalmars-d]

I added support for const and immutable qualifiers to Checked:

https://github.com/dlang/phobos/pull/4613/commits/be23323dc6d510503462ee11dbdb26b7af61aa61

It took 80 lines, which is only some 3% of the code base. Most 
aggravation was in fact caused by opBinary not accepting the "this" 
template parameter. @Walter does that qualify as a bug?

By this I am asking for std.experimental.checkedint to be considered for 
inclusion in Phobos.

Code: https://github.com/dlang/phobos/pull/4613
Docs: 
http://dtest.thecybershadow.net/artifact/website-b243266be0bd9e2a1d56da2d9e76c9e44689e1dd-17113e48d3b1f18b74e0fea1fa8e64ab/web/phobos-prerelease/std_experimental_checkedint.html
Dox: 
http://dtest.thecybershadow.net/artifact/website-b243266be0bd9e2a1d56da2d9e76c9e44689e1dd-17113e48d3b1f18b74e0fea1fa8e64ab/web/library-prerelease/std/experimental/checkedint.html


Thanks,

Andrei