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_bg.d.ts

@@ -0,0 +1,12 @@
+/* tslint:disable */
+/* eslint-disable */
+export function worker_initializer(): number;
+export function start_main_thread(a: number): void;
+export function start_worker_thread(): void;
+export function optimise(a: number, b: number, c: number, d: number): void;
+export function malloc(a: number): number;
+export function free(a: number): void;
+export const __wbindgen_export_0: WebAssembly.Memory;
+export function __wbindgen_malloc(a: number): number;
+export function __wbindgen_free(a: number, b: number): void;
+export function __wbindgen_start(): void;