Somehow give a warning when trying to assign a size_t/ptrdiff_t to int or uint

[WebFreak001 via Digitalmars-d]

It's annoying to fix all these `int index = 
str.indexOf("something")` to size_t/ptrdiff_t because you started 
writing the code thinking that indexOf returns an integer even 
though it returns a ptrdiff_t. When porting code from 32bit to 
64bit you need to fix all these lines which can quickly become 
quite complex from functions where you don't expect them to 
return size_t or ptrdiff_t

Adding a warning when trying to do something like `int i = 
funcReturningPtrdiff();` would make porting easier because you 
would already spot the issues on the OS/architecture you are 
working on. This also affects porting windows to linux as DMD on 
windows uses 32bit by default and on linux it uses the 
architecture you downloaded as default.