Revert "Always set #![no_std]"

This reverts commit 4a53c9b.

This caused a build error on Rust 1.40.

```
error[E0433]: failed to resolve: could not find `poll_with_tls_context` in `future`

error[E0433]: failed to resolve: could not find `from_generator` in `future`

error: aborting due to 2 previous errors
```

Author: taiki-e

src/future.rs

@@ -54,7 +54,6 @@ use core::task::{Context, Poll};
 pub fn block_on<T>(future: impl Future<Output = T>) -> T {
     use std::cell::RefCell;
     use std::task::Waker;
-    use std::thread_local;
 
     use parking::Parker;
     use waker_fn::waker_fn;

src/io.rs

@@ -20,18 +20,14 @@ pub use std::io::{Error, ErrorKind, Result, SeekFrom};
 #[doc(no_inline)]
 pub use futures_io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite};
 
-use std::boxed::Box;
 use std::cmp;
 use std::fmt;
 use std::future::Future;
 use std::io::{IoSlice, IoSliceMut};
 use std::mem;
 use std::pin::Pin;
-use std::string::String;
 use std::sync::{Arc, Mutex};
 use std::task::{Context, Poll};
-use std::vec;
-use std::vec::Vec;
 
 use futures_core::stream::Stream;
 use pin_project_lite::pin_project;

src/lib.rs

@@ -22,12 +22,9 @@
 //! ```
 
 #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
-#![no_std]
+#![cfg_attr(not(feature = "std"), no_std)]
 #![allow(clippy::needless_borrow)] // suggest code that doesn't work on MSRV
 
-#[cfg(feature = "std")]
-extern crate std;
-
 #[cfg(feature = "std")]
 #[doc(no_inline)]
 pub use crate::io::{