chore: use traits in noir-protocol-circuits (#3832)

Simple use of traits in `noir-protocol-circuits` and `aztec-nr` when
possible.

Missing: Serialize/Deserialize due to:
noir-lang/noir#3471

Renamed ::default() to ::empty(), since it better conveys meaning for
our use cases (open to discussion).

aztec/src/note/note_header.nr

@@ -1,4 +1,5 @@
 use dep::protocol_types::address::AztecAddress;
+use dep::protocol_types::traits::Empty;
 
 struct NoteHeader {
     contract_address: AztecAddress,
@@ -9,12 +10,14 @@ struct NoteHeader {
     is_transient: bool,
 }
 
+impl Empty for NoteHeader {
+    fn empty() -> Self {
+        NoteHeader { contract_address: AztecAddress::zero(), nonce: 0, storage_slot: 0, is_transient: false }
+    }
+}
+
 impl NoteHeader {
     pub fn new(contract_address: AztecAddress, nonce: Field, storage_slot: Field) -> Self {
         NoteHeader { contract_address, nonce, storage_slot, is_transient: false }
     }
-
-    pub fn empty() -> Self {
-        NoteHeader { contract_address: AztecAddress::zero(), nonce: 0, storage_slot: 0, is_transient: false }
-    }
 }

safe-math/src/safe_u120.nr

@@ -1,7 +1,18 @@
+use dep::std::ops::Eq;
+
 struct SafeU120 {
     value: u120,
 }
 
+impl Eq for SafeU120 {
+    fn eq(
+        self: Self,
+        other: Self
+    ) -> bool {
+        self.value == other.value
+    }
+}
+
 impl SafeU120 {
     pub fn min() -> Self {
         Self {
@@ -34,13 +45,6 @@ impl SafeU120 {
         self.value == 0
     }
 
-    pub fn eq(
-        self: Self,
-        other: Self
-    ) -> bool {
-        self.value == other.value
-    }
-
     pub fn lt(self: Self, other: Self) -> bool {
         self.value < other.value
     }