Optimize input reading and output writing

This commit aims to fix issue #20.

Use the Emscripten FS.writeFile API for accepting XML input files,
instead of the createDataFile and especially the intArrayFromString
function. Those were inherited from the parent upstream project, but
this writeFile API seems to be simpler to use and performs better.

The bigger fix, though, is in the output side, as pushing one piece of
stdout (I guess it was pushing one byte at a time?) caused the
stdoutBuffer array to eventually grow so large that it'd throw

> RangeError [Error]: Invalid array length

when the output was very big, like when normalizing a big input XML, as
described in #20.

Here, too, we can switch to the print/printErr APIs, which seem to be not
only simpler but also more resilient to the input size growing.

src/worker-post.js

@@ -3,18 +3,14 @@
 	const {parentPort} = require('worker_threads');
 	// #endif
 
-	function bytesToUtf8(buffer) {
-		return new TextDecoder().decode(Uint8Array.from(buffer));
-	}
-
-	const stdoutBuffer = [];
-	const stderrBuffer = [];
+	let stdout = '';
+	let stderr = '';
 
 	function onExit(exitCode) {
 		const message = {
 			exitCode,
-			stdout: bytesToUtf8(stdoutBuffer),
-			stderr: bytesToUtf8(stderrBuffer),
+			stdout,
+			stderr,
 		};
 		// #ifdef node
 		parentPort.postMessage(message);
@@ -37,8 +33,17 @@
 		Module({
 			inputFiles: data.inputFiles,
 			arguments: data.args,
-			stderr: stderrBuffer.push.bind(stderrBuffer),
-			stdout: stdoutBuffer.push.bind(stdoutBuffer),
+			// TODO: We could eagerly start sending stdout to the parent thread while
+			// waiting for more. Or we could probably use some other, more efficient
+			// Emscripten API for output communication in the first place.
+			// But this seems to work fine for now, better than pushing the stdout
+			// values to an array.
+			print(text) {
+				stdout += text;
+			},
+			printErr(text) {
+				stderr += text;
+			},
 			onExit,
 			wasmMemory,
 			// #ifdef browser

src/worker-pre.js

@@ -1,5 +1,5 @@
 Module['preRun'] = function () {
 	Module['inputFiles'].forEach(function(inputFile) {
-		FS.createDataFile('/', inputFile['fileName'], intArrayFromString(inputFile['contents']), true, true);
+		FS.writeFile('/' + inputFile['fileName'], inputFile['contents']);
 	});
 };