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/pkg-parallel/oxipng.d.ts

@@ -0,0 +1,46 @@
+/* tslint:disable */
+/* eslint-disable */
+/**
+* @returns {any} 
+*/
+export function worker_initializer(): any;
+/**
+* @param {number} num 
+*/
+export function start_main_thread(num: number): void;
+/**
+*/
+export function start_worker_thread(): void;
+/**
+* @param {Uint8Array} data 
+* @param {number} level 
+* @returns {Uint8Array} 
+*/
+export function optimise(data: Uint8Array, level: number): Uint8Array;
+
+export type InitInput = RequestInfo | URL | Response | BufferSource | WebAssembly.Module;
+
+export interface InitOutput {
+  readonly worker_initializer: () => number;
+  readonly start_main_thread: (a: number) => void;
+  readonly start_worker_thread: () => void;
+  readonly optimise: (a: number, b: number, c: number, d: number) => void;
+  readonly malloc: (a: number) => number;
+  readonly free: (a: number) => void;
+  readonly __wbindgen_export_0: WebAssembly.Memory;
+  readonly __wbindgen_malloc: (a: number) => number;
+  readonly __wbindgen_free: (a: number, b: number) => void;
+  readonly __wbindgen_start: () => void;
+}
+
+/**
+* If `module_or_path` is {RequestInfo} or {URL}, makes a request and
+* for everything else, calls `WebAssembly.instantiate` directly.
+*
+* @param {InitInput | Promise<InitInput>} module_or_path
+* @param {WebAssembly.Memory} maybe_memory
+*
+* @returns {Promise<InitOutput>}
+*/
+export default function init (module_or_path?: InitInput | Promise<InitInput>, maybe_memory: WebAssembly.Memory): Promise<InitOutput>;
+