Add monotonic check interface

src/merge.rs

@@ -50,6 +50,7 @@ pub trait MergeableTy: fmt::Debug + WithLen + ByteEstimate + Unpin + Sized {
     fn tss_for_testing(&self) -> VecDeque<TsNano>;
     fn drain_into(&mut self, dst: &mut Self, range: Range<usize>) -> DrainIntoDstResult;
     fn drain_into_new(&mut self, range: Range<usize>) -> DrainIntoNewResult<Self>;
+    fn is_strict_monotonic(&self) -> bool;
     fn is_consistent(&self) -> bool;
 }
 
@@ -63,5 +64,6 @@ pub trait MergeableDyn: fmt::Debug + WithLen + ByteEstimate + Unpin + AsAnyMut {
     fn drain_into(&mut self, dst: &mut dyn MergeableDyn, range: Range<usize>)
     -> DrainIntoDstResult;
     fn drain_into_new(&mut self, range: Range<usize>) -> DrainIntoNewDynResult;
+    fn is_strict_monotonic(&self) -> bool;
     fn is_consistent(&self) -> bool;
 }