AK: Define our own concept of "trivially serializable"

While at it, rename the `read_trivial_value` and `write_trivial_value`
functions to `read_value` and `write_value` respectively, since we'll
add compatibility for non-trivial types down the line.

Userland/Libraries/LibDNS/PacketHeader.h

@@ -97,3 +97,8 @@ private:
 static_assert(sizeof(PacketHeader) == 12);
 
 }
+
+template<>
+struct AK::Traits<DNS::PacketHeader> : public AK::GenericTraits<DNS::PacketHeader> {
+    static constexpr bool is_trivially_serializable() { return true; }
+};