`once_cell` provides a neat way of initializing lazy singletons without
macro. This PR use `sync::Lazy` to streamline same pattern proposed in
related rust RFC.
Resolve#406
Previously all of the into_raw_fd implementations only returns a copy of
the inner RawFd, while still holding the ownership of the file
descriptor when returning for into_raw_fd. Since `self` is dropped at
the end of into_raw_fd, the returned file descriptor will actually be
closed, render the function unuseable.
The patch makes sure that into_raw_fd actually takes the ownership of
the file descriptor all the way from the inner IoHandle. To achieve
this, I have to use an Option in IoHandle to store the I/O source. It's
not pretty, but I cannot come up with a better way.
* Add future::poll_fn
* Replace all uses of poll_fn with the new one
* Remove some uses of futures
* Simplify ReadDir and DirEntry
* Remove some use of futures from File
* Use futures subcrates
* Fix imports in docs
* Remove futures-util dependency
* Remove futures-executor-preview
* Refactor
* Require more features in the futures-preview crate
* Implement an async version of ToSocketAddrs
* fix documentation issue
* genius hack: pretending to be `impl Future`
* replace `std::net::ToSocketAddrs` with `async-std::net::ToSocketAddrs`
* Move unit tests into the tests directory
* Stylistic changes
* Remove re-exports in async_std::net
* fix broken link
* some mirror changes
* remove unnecessary format
* migrate: `std::net::ToSocketAddrs` -> `async_std::net::ToSocketAddrs`
* fix typo(tutorial)
* remove unnecessary type bound
* lifetime for future
