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*`.
This moves the wallet structs that are needed elsewhere (for instance,
in the device drivers) from `tools::wallet2::whatever` to
`wallet::whatever`, allowing them to be included individually via
wallet/whatever.h without needing to include the entire wallet2.h.
What was particularly problematic here is that you couldn't even forward
declare them because they were nested inside the wallet2 class, but
instead had to include the full wallet2.h.
