Add support for parsing submodules

[pswaminathan]

No description provided.

[cgrindel]

Sorry for the radio silence on this end. I am out on vacation for the next week and a half. I will take a look at this PR when I am back online.

[cgrindel]

@pswaminathan I am so sorry for the delay in reviewing this. It fell off of my radar. Thank you for sending it.

I think that the submodule parsing should run through the same parsing code as the module parsing. Since we cannot rely on recursive functions, I am thinking that we implement it like so:

1. Extract the submodule tokens,
2. Return unprocessed submodule tokens as a declaration,
3. The top-level module function detects the unprocessed submodule declaration and runs it through the module processing

What do you think? I am working on an implementation. I will send it out for review when I have something that works.

[cgrindel]

@pswaminathan I added my first pass at submodule parsing in #169. If that looks reasonable to you, do you think we should close this PR?