Use impl_op_for_references for block height/interval

We have a new macro for implementing ops with a bunch of reference
combos. Lets use it for block `Height` and `Interval`.

This patch is strictly additive.

units/src/block.rs

@@ -171,6 +171,7 @@ impl fmt::Display for TooBigForRelativeBlockHeightError {
 #[cfg(feature = "std")]
 impl std::error::Error for TooBigForRelativeBlockHeightError {}
 
+crate::internal_macros::impl_op_for_references! {
     // height - height = interval
     impl ops::Sub<BlockHeight> for BlockHeight {
         type Output = BlockInterval;
@@ -220,6 +221,7 @@ impl ops::Sub<BlockInterval> for BlockInterval {
             BlockInterval::from_u32(height)
         }
     }
+}
 
 impl ops::AddAssign<BlockInterval> for BlockInterval {
     fn add_assign(&mut self, rhs: BlockInterval) { self.0 = self.to_u32() + rhs.to_u32(); }