Inline checked mul function back into weight module

A while back we move all the 'fee' stuff into a separate module
because I thought it would help with clarity - I was wrong.

Move the checked mul function back into the `weight` module on the main
`Weight` impl block.

Internal change only - code move.

units/src/fee.rs

@@ -181,20 +181,6 @@ crate::internal_macros::impl_op_for_references! {
     }
 }
 
-impl Weight {
-    /// Checked fee rate multiplication.
-    ///
-    /// Computes the absolute fee amount for a given [`FeeRate`] at this weight. When the resulting
-    /// fee is a non-integer amount, the amount is rounded up, ensuring that the transaction fee is
-    /// enough instead of falling short if rounded down.
-    ///
-    /// Returns [`None`] if overflow occurred.
-    #[must_use]
-    pub const fn checked_mul_by_fee_rate(self, fee_rate: FeeRate) -> Option<Amount> {
-        fee_rate.checked_mul_by_weight(self)
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;

units/src/weight.rs

@@ -10,7 +10,7 @@ use arbitrary::{Arbitrary, Unstructured};
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
-use crate::CheckedSum;
+use crate::{Amount, CheckedSum, FeeRate};
 
 /// The factor that non-witness serialization data is multiplied by during weight calculation.
 pub const WITNESS_SCALE_FACTOR: usize = 4;
@@ -162,6 +162,18 @@ impl Weight {
             None => None,
         }
     }
+
+    /// Checked fee rate multiplication.
+    ///
+    /// Computes the absolute fee amount for a given [`FeeRate`] at this weight. When the resulting
+    /// fee is a non-integer amount, the amount is rounded up, ensuring that the transaction fee is
+    /// enough instead of falling short if rounded down.
+    ///
+    /// Returns [`None`] if overflow occurred.
+    #[must_use]
+    pub const fn checked_mul_by_fee_rate(self, fee_rate: FeeRate) -> Option<Amount> {
+        fee_rate.checked_mul_by_weight(self)
+    }
 }
 
 /// Alternative will display the unit.