disabling unary "-" for unsigned types

[Walter Bright]

dsimcha wrote:
> The more you bring up features to give the axe to, the more I find it funny that,
> not being a very good language lawyer, I wasn't aware that half of these features
> existed in the first place.

D is a very ambitious language, and we are definitely shooting for the 
stars with it. That means that there are many features that missed or 
otherwise failed to find a target. So we're doing a little pruning for 
D2, and a big part of posting these possible prunes here is to make sure 
we aren't missing an important use case for them.

One problem C/C++ has is there are several failed features in them that 
the C/C++ community has repeatedly tried to excise, but they just won't 
die. Trigraphs are an example.