relative locktime: introduce is_* methods to check units

Copy these from absolute::LockTime. While we are at it, make the
functions in absolute::LockTime const.

bitcoin/src/blockdata/locktime/absolute.rs

@@ -176,7 +176,7 @@ impl LockTime {
 
     /// Returns true if this lock time value is a block height.
     #[inline]
-    pub fn is_block_height(&self) -> bool {
+    pub const fn is_block_height(&self) -> bool {
         match *self {
             LockTime::Blocks(_) => true,
             LockTime::Seconds(_) => false,
@@ -185,7 +185,7 @@ impl LockTime {
 
     /// Returns true if this lock time value is a block time (UNIX timestamp).
     #[inline]
-    pub fn is_block_time(&self) -> bool { !self.is_block_height() }
+    pub const fn is_block_time(&self) -> bool { !self.is_block_height() }
 
     /// Returns true if this timelock constraint is satisfied by the respective `height`/`time`.
     ///

bitcoin/src/blockdata/locktime/relative.rs

@@ -21,7 +21,7 @@ pub use units::locktime::relative::{Height, Time, TimeOverflowError};
 
 /// A relative lock time value, representing either a block height or time (512 second intervals).
 ///
-/// Used for sequence numbers (`nSequence` in Bitcoin Core and [`crate::Transaction::TxIn::sequence`]
+/// Used for sequence numbers (`nSequence` in Bitcoin Core and [`crate::TxIn::sequence`]
 /// in this library) and also for the argument to opcode 'OP_CHECKSEQUENCEVERIFY`.
 ///
 /// ### Note on ordering
@@ -141,6 +141,23 @@ impl LockTime {
         }
     }
 
+    /// Returns true if both lock times use the same unit i.e., both height based or both time based.
+    #[inline]
+    pub const fn is_same_unit(&self, other: LockTime) -> bool {
+        matches!(
+            (self, other),
+            (LockTime::Blocks(_), LockTime::Blocks(_)) | (LockTime::Time(_), LockTime::Time(_))
+        )
+    }
+
+    /// Returns true if this lock time value is in units of block height.
+    #[inline]
+    pub const fn is_block_height(&self) -> bool { matches!(*self, LockTime::Blocks(_)) }
+
+    /// Returns true if this lock time value is in units of time.
+    #[inline]
+    pub const fn is_block_time(&self) -> bool { !self.is_block_height() }
+
     /// Returns true if this [`relative::LockTime`] is satisfied by either height or time.
     ///
     /// # Examples