Add unchecked variants to Amount and SignedAmount

The checked variants have worse performance than the unchecked variants
due to the additional branching operations.  To improve performance where
overflow is either not possible or not a concern, unchecked variants
of Amount and SignedAmount are introduced.

units/src/amount.rs

@@ -864,6 +864,22 @@ impl Amount {
     /// Returns [None] if overflow occurred.
     pub fn checked_rem(self, rhs: u64) -> Option<Amount> { self.0.checked_rem(rhs).map(Amount) }
 
+    /// Unchecked addition.
+    /// This is the unsafe variant of addition.  Overflow instead of panic.
+    pub fn unchecked_add(self, rhs: Amount) -> Amount {
+        Self(self.0 + rhs.0)
+    }
+
+    /// Unchecked subtraction.
+    /// This is the unsafe variant of subtraction.  Overflow instead of panic.
+    pub fn unchecked_sub(self, rhs: Amount) -> Amount {
+        Self(self.0 - rhs.0)
+    }
+
+    /// Unchecked multiplication.
+    /// This is the unsafe variant of multiplication.  Overflow instead of panic.
+    pub fn unchecked_mul(self, rhs: Amount) -> Amount { Self(self.0 * rhs.0) }
+
     /// Convert to a signed amount.
     pub fn to_signed(self) -> Result<SignedAmount, ParseAmountError> {
         if self.to_sat() > SignedAmount::MAX.to_sat() as u64 {
@@ -1229,6 +1245,22 @@ impl SignedAmount {
         self.0.checked_rem(rhs).map(SignedAmount)
     }
 
+    /// Unchecked addition.
+    /// This is the unsafe variant of addition.  Overflow instead of panic.
+    pub fn unchecked_add(self, rhs: SignedAmount) -> SignedAmount {
+        Self(self.0 + rhs.0)
+    }
+
+    /// Unchecked subtraction.
+    /// This is the unsafe variant of subtraction.  Overflow instead of panic.
+    pub fn unchecked_sub(self, rhs: SignedAmount) -> SignedAmount {
+        Self(self.0 - rhs.0)
+    }
+
+    /// Unchecked multiplication.
+    /// This is the unsafe variant of multiplication.  Overflow instead of panic.
+    pub fn unchecked_mul(self, rhs: SignedAmount) -> SignedAmount { Self(self.0 * rhs.0) }
+
     /// Subtraction that doesn't allow negative [SignedAmount]s.
     /// Returns [None] if either [self], `rhs` or the result is strictly negative.
     pub fn positive_sub(self, rhs: SignedAmount) -> Option<SignedAmount> {