refactor(unit): update function signatures for consistency and clarity

Author: bobzhang

unit/README.mbt.md

@@ -22,7 +22,7 @@ test "unit construction" {
   
   // Via default constructor
   let u2 = @unit.default()
-  
+  fn println(_ : String) {}
   // All unit values are identical
   inspect(u1 == u2, content="true")
   
@@ -45,7 +45,7 @@ Functions that return `Unit` are typically called for their side effects:
 ```moonbit
 test "side effect patterns" {
   let numbers = [1, 2, 3, 4, 5]
-  
+  fn println(_ : Int) {}
   // Processing for side effects (printing, logging, etc.)
   let processing_result = numbers.fold(init=(), fn(_acc, n) {
     // Simulated side effect

unit/unit.mbt

@@ -13,20 +13,17 @@
 // limitations under the License.
 
 ///|
-pub fn to_string(self : Unit) -> String {
-  let _ = self
+pub fn Unit::to_string(_ : Self) -> String {
   "()"
 }
 
 ///|
-pub impl Hash for Unit with hash(self) {
-  let _ = self
+pub impl Hash for Unit with hash(_) -> Int {
   0
 }
 
 ///|
-pub impl Hash for Unit with hash_combine(self, hasher) -> Unit {
-  let _ = self
+pub impl Hash for Unit with hash_combine(_, hasher) -> Unit {
   hasher.combine_unit()
 }
 
@@ -42,6 +39,6 @@ pub fn default() -> Unit {
 }
 
 ///|
-pub impl Compare for Unit with compare(_self, _other) {
+pub impl Compare for Unit with compare(_, _) {
   0
 }