Implement support for STL model loading/deserialization

[themkat]

Uses stl_io to parse and implement a very simple STL parser. The original STL model format is quite minimal compared to other formats, so I assumed there was only one object in the "scene". Have not experienced it any other way yet, though with a very minimal experience with these formats used for 3D printing. Test cases cover both ascii and binary formats, so they will do basic verification if we ever want to change stl library (or write out own).

Fixes #7

Threw it into the lights demo in the three-d repo, and it seems to work okay:

(that demo has point lights, which is the little dots floating around Suzanne)

EDIT: Notice that the build fails due to missing trait in the web version. I'm on it. Never really use web thingys,

[themkat]

Thanks for fixing @asny ! 😸 I just found that error mapping while debugging just now myself. Did not notice it before 😱

[asny]

No problem, that was a small fix 🙂 The code looks good, thanks a lot for the contribution 🙏