Move Int parsing to i32 for #543 (#700)

Note this doesn't support other sizes or types, it just errors on them.

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

executable_semantics/testdata/generic_function2.carbon

@@ -6,6 +6,6 @@ fn fst[T:! Type](x: T, y: T) -> T {
   return x;
 }
 
-fn main() -> Int {
+fn main() -> i32 {
   return fst(0, 1);
 }