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/tests/task_blocking.rs

@@ -1,5 +1,5 @@
 #![warn(rust_2018_idioms)]
-#![cfg(feature = "full")]
+#![cfg(all(feature = "full", not(target_os = "wasi")))] // Wasi doesn't support threads
 
 use tokio::{runtime, task};
 use tokio_test::assert_ok;