AK: Add a BinarySearch template implementation

binary_search takes a haystack, a size, a needle and a compare function.
The compare function should return negative if a < b, positive if a > b
and 0 if a == b. The "sane default" compare function is integral_compare
which implements this with subtraction a - b.
binary_search returns a pointer to a matching element, NOT necessarily
the FIRST matching element. It returns a nullptr if the element was not
found.

This patch includes tests for binary_search.

AK/BinarySearch.h

@@ -0,0 +1,32 @@
+#pragma once
+
+namespace AK {
+
+template<typename T>
+int integral_compare(const T& a, const T& b)
+{
+    return a - b;
+}
+
+template<typename T, typename Compare>
+T* binary_search(T* haystack, size_t haystack_size, const T& needle, Compare compare = integral_compare)
+{
+    int low = 0;
+    int high = haystack_size - 1;
+    while (low <= high) {
+        int middle = (low + high) / 2;
+        int comparison = compare(needle, haystack[middle]);
+        if (comparison < 0)
+            high = middle - 1;
+        else if (comparison > 0)
+            low = middle + 1;
+        else
+            return &haystack[middle];
+    }
+
+    return nullptr;
+}
+
+}
+
+using AK::binary_search;