Auto merge of #51263 - cramertj:futures-in-core, r=aturon

Add Future and task system to the standard library

This adds preliminary versions of the `std::future` and `std::task` modules in order to unblock development of async/await (https://github.com/rust-lang/rust/issues/50547). These shouldn't be considered as final forms of these libraries-- design questions about the libraries should be left on https://github.com/rust-lang/rfcs/pull/2418. Once that RFC (or a successor) is merged, these APIs will be adjusted as necessary.

r? @aturon

src/libstd/lib.rs

@@ -261,6 +261,7 @@
 #![feature(float_from_str_radix)]
 #![feature(fn_traits)]
 #![feature(fnbox)]
+#![feature(futures_api)]
 #![feature(hashmap_internals)]
 #![feature(heap_api)]
 #![feature(int_error_internals)]
@@ -282,6 +283,7 @@
 #![feature(panic_internals)]
 #![feature(panic_unwind)]
 #![feature(peek)]
+#![feature(pin)]
 #![feature(placement_new_protocol)]
 #![feature(prelude_import)]
 #![feature(ptr_internals)]
@@ -460,6 +462,20 @@ pub use core::u128;
 #[stable(feature = "core_hint", since = "1.27.0")]
 pub use core::hint;
 
+#[unstable(feature = "futures_api",
+           reason = "futures in libcore are unstable",
+           issue = "50547")]
+pub mod task {
+    //! Types and Traits for working with asynchronous tasks.
+    pub use core::task::*;
+    pub use alloc_crate::task::*;
+}
+
+#[unstable(feature = "futures_api",
+           reason = "futures in libcore are unstable",
+           issue = "50547")]
+pub use core::future;
+
 pub mod f32;
 pub mod f64;