Send feature flag about mlua HOT 2 OPEN

It adds Send requirements to Rust functions and userdata objects, which is not always desirable. Also don't be confused with Sync.

PS. Next mlua version will have send feature flag replaced with sync to allow easy integration with multithreading code.

from mlua.

Added the send and async features to Cargo.toml, then placed the Lua object into the BoxFeature. Compiler prompted an error:

*mut c_void cannot be shared between threads safely.

After reviewing the code, should we modify *mut c_void to AtomicPtr?

like:

pub struct LuaInner {
    state: AtomicPtr<ffi::lua_State>,
    // replace main_state: *mut ffi::lua_State,
    main_state: AtomicPtr<ffi::lua_State>,
    extra: Arc<UnsafeCell<ExtraData>>,
}

from mlua.