Re-export task::spawn and task::spawn_blocking at top-level

These are fundamental operations that crates will use constantly; make
it possible to write `async_std::spawn` or `async_std::spawn_blocking`
to make examples and tests easier to write without requiring as many
`use` lines.

Author: joshtriplett

src/lib.rs

@@ -318,6 +318,11 @@ cfg_default! {
     pub mod net;
     #[cfg(not(target_os = "unknown"))]
     pub(crate) mod rt;
+
+    #[cfg(not(target_os = "unknown"))]
+    pub use std::task::spawn;
+    #[cfg(not(target_os = "unknown"))]
+    pub use std::task::spawn_blocking;
 }
 
 cfg_unstable! {