This week I posted on here about my 'Reserved' class that works very
similarly to 'boost::synchronized_value', and today I was thinking that I
would change the mutex to a shared_mutex, and add a new method,
'ReserveReadOnly' to go alongside the 'Reserve'.

But then I thought that this could malfunction if the class contained any
mutable members.

So I think in <type_traits>, we should have 'std::contains_mutable'.