Bring neovim up to date with recent libuv changes

As of v0.11.23 libuv's uv_timer_cb, uv_async_cb, uv_prepare_cb, uv_check_cb and
uv_idle_cb no longer require a status parameter (this went unused in the first
place).

Bump third-party dependency `libuv` up to 0.11.23 and remove the extra
parameters from the callbacks.