checkedint DUB package updated for the new integer promotion rules

[tsbockman]

I have updated my checkedint DUB package to work with newer 
compilers:
     https://code.dlang.org/packages/checkedint
There are two significant breaking changes as a part of this:

* The minimum compiler frontend version is now 2.071.

* If the `-preview=intpromote` compiler option is supplied, the 
return types of `safeOp.unary`, `safeOp.abs`, and the 
corresponding `SafeInt` methods change to match the new behavior 
of the built-in types. See the changelog for D 2.078:
     https://dlang.org/changelog/2.078.0.html#fix16997

I fixed all of the deprecation warnings that I could, but there 
are a few that can only reasonably be removed by enabling 
`-preview=intpromote`. These should go away naturally once 
`intpromote` becomes the default.

If anyone is actually using this package, please let me know 
either here or on GitHub if there's anything else I need to fix.