Expose valid (min, max) difficulty transition thresholds

Once `U256` was made private, we lost the ability to check whether a
valid difficulty transition was made in the chain, since `Target`
no longer exposes any arithmetic operations.

bitcoin/src/pow.rs

@@ -234,6 +234,34 @@ impl Target {
     /// [`difficulty`]: Target::difficulty
     #[cfg_attr(all(test, mutate), mutate)]
     pub fn difficulty_float(&self) -> f64 { TARGET_MAX_F64 / self.0.to_f64() }
+
+    /// Computes the minimum valid [`Target`] threshold allowed for a block in which a difficulty
+    /// adjustment occurs. Note that the result is bounded by [`Target::MAX`].
+    ///
+    /// The difficulty can only decrease or increase by a factor of 4 max on each difficulty
+    /// adjustment period.
+    pub fn min_difficulty_transition_threshold(&self) -> Self {
+        let min = Self(self.0 >> 2);
+        if min > Self::MAX {
+            Self::MAX
+        } else {
+            min
+        }
+    }
+
+    /// Computes the maximum valid [`Target`] threshold allowed for a block in which a difficulty
+    /// adjustment occurs. Note that the result is bounded by [`Target::MAX`].
+    ///
+    /// The difficulty can only decrease or increase by a factor of 4 max on each difficulty
+    /// adjustment period.
+    pub fn max_difficulty_transition_threshold(&self) -> Self {
+        let max = Self(self.0 << 2);
+        if max > Self::MAX {
+            Self::MAX
+        } else {
+            max
+        }
+    }
 }
 do_impl!(Target);