If it isn't a CONS, we return NIL instead of failing. This way, we
can use it in our general iteration functions. Eventually the actual
runtime would use this as well. The macros are mostly for internal
use to do assignment etc.
I'm going to implement a normal stable vector instead of an inline
vector so we have both options for use. I think that's smarter than
just sticking to one.
We can register memory we've allocated onto the heap into a ~sys_t~
instance for examination (read: garbage collection) later. We can
also add items to the symbol table it has internally.
