If the inner type of a channel isn't clonable, an attempt to `derive(Clone)`
for a struct, containing this channel, complains:

```plain
299 |     #[derive(Clone)]
    |              ----- in this derive macro expansion
300 |     struct MyChannel(fiber::Channel<NonClonable>);
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Clone`
    |                               is not implemented for `NonClonable`
```

This patch makes channels always clonable as internally
they only use `Rc` (which is always clonable too).

- PushIterError
- TuplePushError

opaque lua stack object that can be used to get other stuff

+ add `Index`, `NewIndex` & `Call` traits
+ add `Indexable`, `IndexableRW`, `Callable` structs to work with values
  on lua stack through the `__index`, `__newindex` & `__call`
  metamethods
+ e.g. you can work with `tuple` or other `ffi` values through lua that
  way

the api mirrors that of `std::sync::Mutex` with a couple of excpetions:
- no way to check if the mutex is poisoned, because a panic inside of
  a fiber is impossible to catch for the moment
- `Mutex::try_lock` will abort (in a tarantool debug build) if called
  from a fiber who already holds the lock. Fixing this would require to
  delve into `box_latch_t` internals, which can be changed in future
  versions of tarantool

remove `#[macro_use]` and `extern crate` directives

add `use <crate>::<macro>` directives

+ specify index parts by path
+ create space via builder
+ create index via builder