sample-code: Add an example function to print a number in hexadecimal

sample-code/hex.cl

@@ -0,0 +1,12 @@
+pub fn hex(mut n: u64) {
+    let hex_lut = [
+        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f',
+    ];
+    for xd in 0..16 {
+        let n = n >> (15 - xd) * 4;
+        if n != 0 {
+            print(hex_lut[n & 0xf])
+        }
+    }
+    println()
+}