arrow-buffer: make i256::leading_zeros public and tested (#8964)

# Which issue does this PR close?

- Closes #8965.

# Rationale for this change

`leading_zeros` exists for other integer types, and in wide `i128` as
well. This PR makes it public and tested. It's worth having it as a
public part of the struct.

# What changes are included in this PR?

- Make the function public
- Unit tests
- Function documentatioon

# Are these changes tested?

Yes, via unit tests.

# Are there any user-facing changes?

No

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: theirix

arrow-buffer/src/bigint/mod.rs

@@ -583,7 +583,8 @@ impl i256 {
         self.high.is_positive() || self.high == 0 && self.low != 0
     }
 
-    fn leading_zeros(&self) -> u32 {
+    /// Returns the number of leading zeros in the binary representation of this [`i256`].
+    pub const fn leading_zeros(&self) -> u32 {
         match self.high {
             0 => u128::BITS + self.low.leading_zeros(),
             _ => self.high.leading_zeros(),
@@ -1336,6 +1337,22 @@ mod tests {
         assert!(out.is_finite() && out.is_sign_negative());
     }
 
+    #[test]
+    fn test_leading_zeros() {
+        // Without high part
+        assert_eq!(i256::from(0).leading_zeros(), 256);
+        assert_eq!(i256::from(1).leading_zeros(), 256 - 1);
+        assert_eq!(i256::from(16).leading_zeros(), 256 - 5);
+        assert_eq!(i256::from(17).leading_zeros(), 256 - 5);
+
+        // With high part
+        assert_eq!(i256::from_parts(2, 16).leading_zeros(), 128 - 5);
+        assert_eq!(i256::from_parts(2, i128::MAX).leading_zeros(), 1);
+
+        assert_eq!(i256::MAX.leading_zeros(), 1);
+        assert_eq!(i256::from(-1).leading_zeros(), 0);
+    }
+
     #[test]
     fn test_trailing_zeros() {
         // Without high part