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_function_tuple_map.carbon

@@ -6,8 +6,8 @@ fn map[T:! Type](f: fnty (T) -> T, tuple: (T, T)) -> (T, T) {
   return (f(tuple[0]), f(tuple[1]));
 }
 
-fn inc(x: Int) -> Int { return x + 1; }
+fn inc(x: i32) -> i32 { return x + 1; }
 
-fn main() -> Int {
+fn main() -> i32 {
   return map(inc, (0, 2))[0];
 }