Conversion category for optionals

[grisumbras]

#807 introduced a special case for when an struct contains an optional member. But it is special cased for std::optional. Having a similar behaviour for boost::optional is logical, but implementing it the same way introduces a dependency on Boost.Optional and it's also a bit silly to have several special cases. It appears, we need a special coversion category for optionals. We can look at Boost.Url for ideas on how to implement the check for whether something is an optional.