wasm: initial support for wasm32-wasi target (#4716)

This adds initial, unstable, support for the wasm32-wasi target. Not all of Tokio's
features are supported yet as WASI's non-blocking APIs are still limited.

Refs: tokio-rs/tokio#4827

tokio-util/src/lib.rs

@@ -29,6 +29,7 @@ cfg_codec! {
 }
 
 cfg_net! {
+    #[cfg(not(target_arch = "wasm32"))]
     pub mod udp;
     pub mod net;
 }

tokio-util/src/task/mod.rs

@@ -2,7 +2,9 @@
 
 #[cfg(tokio_unstable)]
 mod join_map;
+#[cfg(not(target_os = "wasi"))]
 mod spawn_pinned;
+#[cfg(not(target_os = "wasi"))]
 pub use spawn_pinned::LocalPoolHandle;
 
 #[cfg(tokio_unstable)]