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

@@ -187,7 +187,7 @@ 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,
@@ -196,7 +196,7 @@ 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

@@ -6,7 +6,7 @@
 //! whether bit 22 of the `u32` consensus value is set.
 //!
 
-use core::{cmp, convert, fmt};
+use core::{cmp, convert, fmt, mem};
 
 #[cfg(all(test, mutate))]
 use mutagen::mutate;
@@ -18,7 +18,7 @@ use crate::Sequence;
 
 /// 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
@@ -138,6 +138,25 @@ impl LockTime {
         Time::from_seconds_ceil(seconds).map(LockTime::Time)
     }
 
+    /// Returns true if both lock times use the same unit i.e., both height based or both time based.
+    #[inline]
+    pub fn is_same_unit(&self, other: LockTime) -> bool {
+        mem::discriminant(self) == mem::discriminant(&other)
+    }
+
+    /// Returns true if this lock time value is in units of block height.
+    #[inline]
+    pub const fn is_block_height(&self) -> bool {
+        match *self {
+            LockTime::Blocks(_) => true,
+            LockTime::Time(_) => false,
+        }
+    }
+
+    /// 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