Add support for Streams API for stdin, stdout and stderr

[brunoais]

I believe the javascript Streams API would provide a huge improvement to I/O handling for the programs compiled with emscripten.

In the case of the stdin, stdout, stderr files, their API could be improved with:

1. FS.init() receiving, for stdin a ReadableStream (in addition to the current way)
2. FS.init() receiving for each of stdout and stderr its own WritableStream

Ideally, these functions would work with arbitrary chunk sizes and the chunks are sent as-is to the program running.

Possibly, emscripten emulated devices would just work this way instead of custom code (I'm not sure about performance here as I don't know the code enough).

For now, I thought the changes and adaptations would be executed in library_fs.js but the code there appears to be incomplete for the purpose of creating devices and pipes.

(Maybe this could also be the basic blocks of a fifo made with mkfifo?)

Is this feasible?