AK: Allow comparing spans of different constness

Otherwise, the following code would not compile:

    constexpr Array<int, 3> array { 4, 5, 6 };
    Vector<int> vector { 4, 5, 6 };

    if (array == vector.span()) { }

We do such comparisons in tests quite a bit. But it currently doesn't
become an issue because of the way EXPECT_EQ copies its input parameters
to non-const locals. In a future patch, that copying will be removed,
and the compiler would otherwise complain about not finding a suitable
comparison operator.

Tests/AK/TestSpan.cpp

@@ -159,3 +159,11 @@ TEST_CASE(contains_slow)
     EXPECT(!span.contains_slow(String {}));
     EXPECT(!span.contains_slow(StringView {}));
 }
+
+TEST_CASE(compare_different_constness)
+{
+    constexpr Array<int, 3> array { 4, 5, 6 };
+    Vector<int> vector { 4, 5, 6 };
+
+    EXPECT_EQ(array, vector.span());
+}