Merge rust-bitcoin#3274: Add Arbitrary to SignedAmount type

3f244db Add Arbitrary to SignedAmount type (yancy)

Pull request description:

  While proptesting, I've found Arbitrary for SingedAmount to be useful.  Would appreciate adding this as well.

ACKs for top commit:
  tcharding:
    ACK 3f244db
  Kixunil:
    ACK 3f244db

Tree-SHA512: f293215505db85998135c75e147f4cb0031a08c7db619b706e6c7d632c6f23783af56992531396aecc6e3da55b107a7f2c7ec4d68a3e5737ebcf4225f306ed29

units/src/amount.rs

@@ -1593,6 +1593,14 @@ impl core::iter::Sum for SignedAmount {
     }
 }
 
+#[cfg(feature = "arbitrary")]
+impl<'a> Arbitrary<'a> for SignedAmount {
+    fn arbitrary(u: &mut Unstructured<'a>) -> arbitrary::Result<Self> {
+        let s = i64::arbitrary(u)?;
+        Ok(SignedAmount(s))
+    }
+}
+
 /// Calculates the sum over the iterator using checked arithmetic.
 pub trait CheckedSum<R>: private::SumSeal<R> {
     /// Calculates the sum over the iterator using checked arithmetic. If an over or underflow would