since 1.1 rmp-serde disallows ignoring fields when deserializing because
in some cases that would lead to incorrect data being deserialized.

for now we'll just freeze the version, but we really should fix our
tests and explicitly forbid such behaviour

Custom implementation of Deserialize needs for deserialization from any case strings.
Set serialization mode to lower case for all schema enums.

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