Improve Performance on Subsequent Builds

[matthewsot]

Building papers with bazel-latex can be noticeably slower than just running pdflatex.

Some of this is certainly overhead in calling bazel, etc., however I think the bigger issue is that we don't cache build artifacts.

So whereas, if you've already built the paper once, you can usually just run pdflatex, we have to run pdflatex -> bibtex -> pdflatex -> pdflatex which blows up the time-to-build.

This should actually be pretty simple to fix. We can just declare the latex.out/* folder as an output of the rule in Bazel. Though there may be some difficulties in loading it back in at the beginning of the rule (sort of a circular dependency?). I'll work on it.