disabling unary "-" for unsigned types

[Walter Bright]

Steven Schveighoffer wrote:
> First, it would work under my rules.  j would be of type int.  Under my 
> rules, negating an unsigned value equates to a signed version of that 
> type.


I've tried to stick with the principle that C code compiled with D will 
either work the same, or will fail with a compiler error message. It's 
very important to avoid "compiles, but produces subtly different 
behavior" for integer numeric code.

The reason for this is there's a lot of debugged, working C code that 
contains rather complicated integer expressions. How/why it works may be 
long since lost information, and having it fail when translated to D 
will cause frustration and distaste for D.

Changing the signedness of a sub-expression will definitely fall into 
this category.