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/interpreter/value.cpp

@@ -192,7 +192,7 @@ void Value::Print(llvm::raw_ostream& out) const {
       out << "Bool";
       break;
     case Value::Kind::IntType:
-      out << "Int";
+      out << "i32";
       break;
     case Value::Kind::TypeType:
       out << "Type";