Virtualize path and file system operations

[chengluyu]

When I make web demos, one of the headaches is that os.Path doesn’t compile when the target is JavaScript. I made a hack on my web demo branch. But the implementation is a bit crappy, especially after merging the latest changes for a few times. Therefore, I want to introduce a path representation and abstract read operations that both supports a real file system or a virtual one (e.g., when the compiler is running on the web).

This PR intends to do the following things.

• Provide an abstraction layer of path and file system operations.
• Call the MLscript compiler from JavaScript.
  • Compile single MLscript source file.
  • Compile multiple MLscript source files.
• Provide a minimal web demo to showcase the ability to compile multiple files.