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/pattern_variable_fail.6c

@@ -4,6 +4,6 @@
 
 fn main() -> Int {
   // error
-  Int : x;
+  Int x;
   return 1;
 }