Tighten up some elements

@ptoomey3  @gregose @bkeepers

Just a couple of things here:

1. In case `calloc` fails for some reason, we'll bail out.
2. Instead of using [Lasem's wrapper to itex2MML](https://github.com/gjtorikian/mathematical/blob/d15af53fc4cf6977758ca747df1848ed21c97524/ext/mathematical/lsmitex.c#L39), we'll just use itex2MML directly. This bypasses awkwardness around going from an `int` size to an `unsigned long` size.

Any other feedback?