Parallel OxiPNG improvements

- Refactor to work around Chromium's issue with postMessage queuing. https://bugs.chromium.org/p/chromium/issues/detail?id=1075645
 - Convert codec code to TypeScript.
 - Make separate parallel and non-parallel builds.
 - Switch to nightly Rust for OxiPNG to allow parallel builds (but also reuse it for regular builds to avoid installing two toolchains).

codecs/oxipng/src/parallel.rs

@@ -0,0 +1,39 @@
+use crossbeam_deque::Injector;
+use once_cell::sync::OnceCell;
+use wasm_bindgen::prelude::*;
+use wasm_bindgen::JsValue;
+
+#[wasm_bindgen]
+extern "C" {
+    #[wasm_bindgen(js_namespace = Array, js_name = of)]
+    fn array_of_2(a: JsValue, b: JsValue) -> JsValue;
+}
+
+static TASKS: OnceCell<Injector<rayon::ThreadBuilder>> = OnceCell::new();
+
+#[wasm_bindgen]
+pub fn worker_initializer() -> JsValue {
+    TASKS.get_or_init(Injector::new);
+    array_of_2(wasm_bindgen::module(), wasm_bindgen::memory())
+}
+
+#[wasm_bindgen]
+pub fn start_main_thread(num: usize) {
+    let tasks = TASKS.get().unwrap();
+
+    rayon::ThreadPoolBuilder::new()
+        .num_threads(num)
+        .spawn_handler(|thread| Ok(tasks.push(thread)))
+        .build_global()
+        .unwrap_throw()
+}
+
+#[wasm_bindgen]
+pub fn start_worker_thread() {
+    let tasks = TASKS.get().unwrap();
+    loop {
+        if let crossbeam_deque::Steal::Success(task) = tasks.steal() {
+            return task.run();
+        }
+    }
+}