AK: Add Array::contains_slow() and ::first_index_of(), with tests :^)

AK/Array.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/Iterator.h>
+#include <AK/Optional.h>
 #include <AK/Span.h>
 #include <AK/StdLibExtras.h>
 #include <AK/TypedTransfer.h>
@@ -119,6 +120,20 @@ struct Array {
         return value;
     }
 
+    bool contains_slow(T const& value) const
+    {
+        return first_index_of(value).has_value();
+    }
+
+    Optional<size_t> first_index_of(T const& value) const
+    {
+        for (size_t i = 0; i < Size; ++i) {
+            if (__data[i] == value)
+                return i;
+        }
+        return {};
+    }
+
     Conditional<Size == 0, Detail::EmptyArrayStorage<T>, T[Size]> __data;
 };