let Ruby string with \0 go freely to the Ruby side

for gh-14
simplify stack_push_string

for gh-14
stick to ffi 1.9.x
update Gemfile.lock
Changed lua_pushlstring signature to use :pointer instead of :string
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).
Add failing test for strings turning into functions when coroutines are around.
Make it explicitly clear that only strings originating in the host language are affected by coroutines.