When targetting macos <10.14 macos won't allow use of anything from
C++17 that throws, such as:
- std::get on a variant
- std::visit
- std::optional::value()
- std::any_cast
This avoids all of these.
For std::get, we either replace with std::get_if (where appropriate), or
else use a `var::get` implementation of std::get added to lokimq (also
updated here). (This `var` namespace is just an `std` alias everywhere
*except* old target macos).
For std::visit, likewise lokimq adds an var::visit implementation for
old macos that we use.
std::optional::value() uses weren't useful anyway as everywhere it calls
them we've already checked that the option has a value, in which case we
can use `*opt` (which doesn't check for contents and throw).
std::any just has to be avoided as far as I can tell, but the one place
we used it is only ever a block, so I just replaced it with a `const
block*`.
A huge amount of this is repetitive:
- `boost::get<T>(variant)` becomes `std::get<T>(variant)`
- `boost::get<T>(variant_ptr)` becomes `std::get_if<T>(variant_ptr)`
- `variant.type() == typeid(T)` becomes `std::holds_alternative<T>(variant)`
There are also some simplifications to visitors using simpler stl
visitors, or (simpler still) generic lambdas as visitors.
Also adds boost serialization serializers for std::variant and
std::optional.
