Various shared bugs

[Jason House]

So, after months of avoiding shared, I decided to see if I could remove all 
my casting away of shared.  It looks like things are still pretty buggy (or 
at least not particularly easy to use).

"is(T : shared)" gives a parse error
"alias shared T U" silently does the wrong thing.  
   (Use "alias shared(T) U" instead)
"xxxx is not callable using argument types ()" is just awful.  Typically, it 
means you're calling a non-shared function with a shared type or a shared 
function with a non-shared type.  I hit into a case where I got that message 
with a shared type and a shared method, but I have mad no attempt to 
reproduce.
The error "Can not implicitly convert expression of type(this) of type 
shared(xxx) to full.name.xxx" can pop up when defining shared this() 
constructor gives no line number where the offending usage occurs.

I've backed out most of my pro-shared changes and will try again in a few 
months :(