make u128 function as const (#92)

* make u128 function as const

* impl iter sum for U128

ontio-std/src/types/num.rs

@@ -1,6 +1,6 @@
-use core::fmt::{Debug, Display, Result};
+use core::fmt::{Debug, Display, Formatter, Result};
+use core::iter::Sum;
 use core::ops::{Add, AddAssign, Div, Mul, Sub, SubAssign};
-use fixed_hash::static_assertions::_core::fmt::Formatter;
 
 #[derive(Clone, Copy, PartialOrd, PartialEq, Eq, Default)]
 pub struct U128(u128);
@@ -16,15 +16,15 @@ impl I128 {
         U128(self.0 as u128)
     }
 
-    pub fn from_le_bytes(bs: [u8; 16]) -> Self {
+    pub const fn from_le_bytes(bs: [u8; 16]) -> Self {
         I128(i128::from_le_bytes(bs))
     }
 
-    pub fn to_le_bytes(self) -> [u8; 16] {
+    pub const fn to_le_bytes(self) -> [u8; 16] {
         self.0.to_le_bytes()
     }
 
-    pub fn raw(self) -> i128 {
+    pub const fn raw(self) -> i128 {
         self.0
     }
 }
@@ -33,27 +33,33 @@ impl U128 {
     pub const fn new(val: u128) -> Self {
         U128(val)
     }
-    pub fn from_le_bytes(bs: [u8; 16]) -> Self {
+    pub const fn from_le_bytes(bs: [u8; 16]) -> Self {
         U128(u128::from_le_bytes(bs))
     }
 
-    pub fn is_zero(self) -> bool {
+    pub const fn is_zero(self) -> bool {
         self.0 == 0
     }
 
-    pub fn to_le_bytes(self) -> [u8; 16] {
+    pub const fn to_le_bytes(self) -> [u8; 16] {
         self.0.to_le_bytes()
     }
 
-    pub fn raw(self) -> u128 {
+    pub const fn raw(self) -> u128 {
         self.0
     }
 
-    pub fn to_i128(self) -> I128 {
+    pub const fn to_i128(self) -> I128 {
         I128(self.0 as i128)
     }
 }
 
+impl Sum for U128 {
+    fn sum<I: Iterator<Item = Self>>(iter: I) -> Self {
+        iter.fold(U128::new(0), Add::add)
+    }
+}
+
 impl Add<U128> for U128 {
     type Output = U128;