Add var::get/var::visit implementations of std::get/std::visit that get
used if compiling for an old macos target, and use those.
The issue is that on a <10.14 macos target Apple's libc++ is missing
std::bad_variant_access, and so any method that can throw it (such as
std::get and std::visit) can't be used. This workaround is ugly, but
such is life when you want to support running on Apple platforms.
On the wire they are just lists, but this lets you put tuples onto and
pull tuples off of the wire. (Also supports std::pair).
Supports direct serialization (via bt_serialize()/bt_deserialize()),
list/dict consumer deserialization, and conversion from a bt_value or
bt_list via a new bt_tuple() function.