AK: Use __builtin_memset() and such to reduce header dependencies

We can use __builtin_memset() without including <string.h>.
This is pretty neat, as it will allow us to reduce the header deps
of AK templates a bit, if applied consistently.

Note that this is an enabling change for an upcoming #include removal.

AK/Optional.h

@@ -157,7 +157,7 @@ private:
         ASSERT(m_has_value);
         return *reinterpret_cast<const T*>(&m_storage);
     }
-    u8 m_storage[sizeof(T)] { 0 };
+    unsigned char m_storage[sizeof(T)] { 0 };
     bool m_has_value { false };
 };