The cast(signed), cast(unsigned) feature proposal

[Mrzlga]

> How about this: if `Foo` is a template with a single type 
> parameter that returns a type, writing `cast(Foo)bar` will be 
> parsed as `cast(Foo!(typeof(bar))bar`.

My proposal has a bad thing about it:

The 'unsigned' and 'signed' keywords can't be as easily extended 
the way that an signed(x) template in the library can: A library 
author could just specialize the signed() template to add support 
for their own numeric type. To do the same with cast(signed) it 
needs to invoke some signed operator on the type, or use your 
solution.

I wanted to deal with the cast(int) x; not showing the intention 
of "signed". If your cast(SomeTemplate) solution can handle this 
as well, then it's better.

I don't actually mind writing signed(x), but I want to think of a 
way to discourage people from writing cast(int) x when they are 
doing nothing but converting unsigned->signed.