Remove : from vars in exec semantics (#504)

Going through tests, funptr1.6c is the main spot I see a real change, I added parens to get around an issue parsing fnty.

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/testdata/experimental_continuation2.6c

@@ -5,7 +5,7 @@
 // Test creating and running a continuation.
 
 fn main() -> Int {
-  var Int: x = 0;
+  var Int x = 0;
   __continuation k {
     x = x + 1;
   }