Ensure WordContext is NoShare.
Added physics engine as well as a few other fixes.

*   Fixed a memory safety issue (pointing diectly into a HashMap) by
    storing references to arena-allocated elements in the HashMap instead.
*   Added nphysics to the package.  Words are now modeled as cuboids.
*   More traditional draw routine (we don't cache contexts to the extent
    that we did, because it turned out not to work well with
    transformations after the fact).
*   Rather than just one box allocated during draw, boxes are spawned
    with the spacebar.
*   Fixed memory leak in truetype-rs (font faces are deallocated, but
    not the library itself; Valgrind caught this while I was looking
    into the memory safety issue).

The new code isn't necessarily well-optimized, so there are likely going
to be improvements down the line.