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

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

Lua now has a type parameter OnDrop, which determines what happens when
this instance is dropped. There are now 3 variants of Lua:
- type TempLua = Lua<on_drop::Close>;    lua_close will be called in drop
- type StaticLua = Lua<on_drop::Ignore>; nothing happens in drop
- type LuaThread = Lua<on_drop::Unref>;  luaL_unref is called in drop

LuaThread can only be created from a StaticLua, so we don't to think
about what happens when the parent state is closed

tarantool::lua_thread should be used instead of working directly with
tarantool::global_lua whenever possible, to avoid poluting the global
lua stack and introducing race conditions when working from different
fibers

rename hlua to tlua to avoid conflicts when publishing the crate to
crates.io.
flatten the repository structure a bit

It uses the underlying tarantool fiber_channel api: a bounded ring
buffer based channel on top of the fiber cooperative multitasking
runtime.

When the join handle get's dropped there's no way to join the fiber
anymore. But if it is left joinable at this point, the tarantool
internals won't be able to recycle it's memory. So we need to make sure
the join handle is joined explicitly at some point in the code.

- defer functionality is now hidden behind the "defer" feature
- added comments explaining which methods do how many yields

There are now 2 types of fibers
* Immediate - a fiber that starts executing immediately
* Deferred  - a fiber that is scheduled for execution
Both of them come in 2 variants, consider the case of Immediate:
* Immediate<F, T>  - has a fiber function that returns a T
* UnitImmediate<F> - has a fiber function that returns nothing. This is
                     an optimized version of Immediate<F, ()>.
Same with Deferred and UnitDeferred.

All of these fibers are joinable. Non-joinable fibers are not supported
yet.

1. Port `box.schema.space.create()` function to `Space::create`.
2. Add tests for `Space::create`.
3. Remove all *.snap and *.xlog files after running tests.

1. Implement box.session.uid and box.session.euid via lua C API.
2. Add seprate module to import Lua and Tarantool C functions via FFI.