setting default args to none rather than whatever default
value allows for easily checking whether the arg was set,
without losing anything (since e.g. calling get_integer on
a none value still returns a 0)
'b' and 'F' were kinda ugly and handled special niches, which
are no longer a thing now that we're defaulting to none
this is not a good api (it prevents extensibility, requires
handling errors twice to be safe - once on user side, once
internally - and so on); just cast it
the user can now define commands '//ivar', '//fvar' and '//svar'
which take identity (which is the variable) as the first argument
followed by any number of user defined arguments with types (one
can use the N argument type to check argument count, and when no
value arguments are provided, print the variable); this allows us
to implement different styles of variable setting without the
interpreter itself knowing about it, as well as ditch all the
stuff with overridden vars and hex vars and whatnot since this is
all specific to the engine and has no place in here
there is still leftover code remaining, which will get cleaned up
afterwards...