v5.0.0-alpha Optimize file input and stdout output

Attempt to fix #20 by using two more performant Emscripten APIs, FS.writeFile instead of Fs.createDataFile+intArrayFromString, and for output Module.print instead of Module.stdout.

I don't think this should be a breaking change, for most people at least, but I'm doing a major version bump anyway because I think maybe the createDataFile vs writeFile change could cause trouble if the input XML had some other encoding than utf-8. FS.writeFile assumes utf-8 when the input is string.

TODO: We now also accept Buffer in the XML file contents, which should be reflected in types. Also, more testing is needed before release, I haven't checked browser usage at all yet.