checkedint call removal

[Vlad Levenfeld via Digitalmars-d]

in/out contracts ought to remain in release builds.

Since debug and non-zero-assert disappear in release, I can just 
use them inside the in/out blocks when I want debug-only 
verification steps.

Compiling away contracts seems redundant, and it would help 
reduce noise to push more verification logic out of function 
bodies and into contracts.