Add ability to write lexicons directly

[JMMackenzie]

Implements direct writes for the .termlex and .doclex files when running ciff2pisa.

[elshize]

Since we're using this code twice, it might be good to create a build_lexicon function (or whatever you feel the good name would be) that takes input txt file and output lexicon path, and does all this: reads, creates vector, writes.

We can actually add it in payload_vector module, and rewrite the test_write test slightly to use it. We'd need to first write the txt file instead of reading it from a file that's in git because of newlines problem on windows; in that case we can remove that file altogether I think; you could also rewrite this code to not explicitly use \n as separator, e.g., use lines on BufReader instance).

I think this might work, though I'm typing it off the top of my head:

let termlex = BufReader::new(File::open(input)?)
    .lines()
    .collect::<Result<PayloadVector, _>>()?;

[JMMackenzie]

Agreed. I have worked the function in but have not yet updated the test case.

[JMMackenzie]

@elshize Is this ready for review/merge?

[elshize]

@JMMackenzie if you're ok with my edits, then go ahead and merge.