Implement Ser+De for Saturating<T>

The serialization implementation is heavily
inspired by the existing trait implentation for
`std::num::Wrapping<T>`.

The deserializing implementation maps input values
that lie outside of the numerical range of the
output type to the `MIN` or `MAX` value of the
output type, depending on the sign of the input
value. This behaviour follows to the `Saturating`
semantics of the output type.

fix #2708

test_suite/tests/test_ser.rs

@@ -8,7 +8,7 @@ use std::cell::RefCell;
 use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use std::ffi::CString;
 use std::net;
-use std::num::Wrapping;
+use std::num::{Saturating, Wrapping};
 use std::ops::Bound;
 use std::path::{Path, PathBuf};
 use std::rc::{Rc, Weak as RcWeak};
@@ -624,6 +624,11 @@ fn test_wrapping() {
     assert_ser_tokens(&Wrapping(1usize), &[Token::U64(1)]);
 }
 
+#[test]
+fn test_saturating() {
+    assert_ser_tokens(&Saturating(1usize), &[Token::U64(1)]);
+}
+
 #[test]
 fn test_rc_dst() {
     assert_ser_tokens(&Rc::<str>::from("s"), &[Token::Str("s")]);