feat(Timestamp): implement AddAssign

Closes #221 .

src/models/blockchain/block/difficulty_control.rs

@@ -589,7 +589,7 @@ mod test {
                 );
                 block_times.push(block_time);
                 let old_timestamp = new_timestamp;
-                new_timestamp = new_timestamp + Timestamp::seconds(block_time.round() as u64);
+                new_timestamp += Timestamp::seconds(block_time.round() as u64);
 
                 difficulty = difficulty_control(
                     new_timestamp,

src/models/blockchain/block/mod.rs

@@ -970,7 +970,7 @@ mod block_tests {
 
                 for i in (0..30).step_by(1) {
                     let duration = i as u64 * multiplier;
-                    now = now + Timestamp::millis(duration);
+                    now += Timestamp::millis(duration);
 
                     let (block, _, _) =
                         make_mock_block(&block_prev, Some(now), a_recipient_address, rng.gen());

src/models/blockchain/transaction/validity/update.rs

@@ -779,7 +779,7 @@ pub(crate) mod test {
             .await
             .unwrap();
         let num_seconds = (0u64..=10).new_tree(&mut test_runner).unwrap().current();
-        updated.kernel.timestamp = updated.kernel.timestamp + Timestamp::seconds(num_seconds);
+        updated.kernel.timestamp += Timestamp::seconds(num_seconds);
 
         UpdateWitness::from_old_transaction(
             old_pw.kernel,
@@ -820,7 +820,7 @@ pub(crate) mod test {
         let mut new_kernel = primitive_witness.kernel.clone();
         new_kernel.mutator_set_hash = new_msa.hash();
 
-        new_kernel.timestamp = new_kernel.timestamp + Timestamp::days(1);
+        new_kernel.timestamp += Timestamp::days(1);
         assert_ne!(
             new_msa, primitive_witness.mutator_set_accumulator,
             "must update mutator set too in order for test to be meaningful"

src/models/proof_abstractions/timestamp.rs

@@ -1,5 +1,6 @@
 use std::fmt::Display;
 use std::ops::Add;
+use std::ops::AddAssign;
 use std::ops::Mul;
 use std::ops::Sub;
 use std::time::SystemTime;
@@ -79,6 +80,12 @@ impl Sub for Timestamp {
     }
 }
 
+impl AddAssign for Timestamp {
+    fn add_assign(&mut self, rhs: Self) {
+        *self = *self + rhs
+    }
+}
+
 impl Mul<usize> for Timestamp {
     type Output = Timestamp;