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/global_variable5.6c

@@ -4,9 +4,9 @@
 
 // Test overshadowing of global variable.
 
-var Int: x = 1;
+var Int x = 1;
 
-fn identity(Int: x) {
+fn identity(Int x) {
   return x;
 }