Add a `origFileContents` optional param to parse()

I'd really like to use coverify for extracting coverage info for my test runner, but I'd like to use it in a context where the source text is not actually a file on disk. (It is actually, but I've already read it into memory in the test runner...so I'd prefer to give coverify the opportunity to not have to read it from disk again).

So this is my first attempt at adding support for passing original file contents into the parse stream builder.
Happy to explore other solutions here too -- but I'm basically just looking for some way give `parse()` the file contents I already have rather than have it go off on its own and read them from disk.

Thoughts?