Deprecate {} without () for function pointers and delegates

[ABrightLight]

It's generally been agreed on in the past that {} without () in 
lambdas causes ambiguity with block statements.
```
void foo (void delegate () dg) {
   dg();
}

void main () {
   auto dg = {1.writeln;}; // Works.

   foo(dg); // function `foo` is not callable using argument types 
`(void function() @safe)` (This is not entirely related but I 
thought I'd point this out as something a bit odd)

   {1.writeln;}(); // Error
}
```
These are the simpler cases where it could be a problem. But I 
recently thought of `lazy`, and how expression-statements are 
coerced to delegates under the hood, but what if you wanted to 
pass a block statement to a function that takes `lazy`?

Anyway, I'd rather not have to juggle these questions in my mind, 
and a simple solution would be to deprecate function literals 
that omit the parentheses for the parameter list. Having to type 
an extra 2 characters is well worth not having to think about the 
edge cases.

Thank you