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. This will be done in a separate PR.

[LPTK]

Nice! But please remove all useless whitespace changes from the diff.

[LPTK]

Nice work!

However, your changes broke diagnostic reporting.

First, the diagnostics are no longer colored in the console.

Second, they are no longer synchronized. For instance, add a bunch of errors in compilation-test files and watch the output get mangled.

Compare with the previous output:

While the Compiling: and /!!!\ Error messages are not synchronized (would be easy to fix), each error is properly reported separately and not interleaved.

[LPTK]

Ok, it mostly LGTM, but please address these extra commnets and what we discussed by messages.