Switch to crossbeam-channel

Still not perfect due to usage of a static global, but this is much cleaner and more efficient thanks to proper blocking of Workers that wait for new messages instead of a manual spin-loop.

codecs/oxipng/src/lib.rs

@@ -1,4 +1,5 @@
 use wasm_bindgen::prelude::*;
+use oxipng::AlphaOptim;
 
 mod malloc_shim;