Add push string to stack support, plus failing unit test re: coroutines

Updated version of  #18

Added 3 different ways to push a string to the stack. At one point, only stack_push_string_unit8 worked. Now all 3 work, perhaps due updating FFI?
Added additional specs to test round-tripping from both sides, as well as verify that bytes after the first \0 aren't also \0 (since this happened during testing).