Add a LanguageModel class that implements the Llama2 architecture via llama2.c and Emscripten

[gohai]

This makes it possible to do inference on toy-models (15M, 42M, 110M parameters) using the Llama2 architecture, as implemented in llama2.c.

The included emscripten build artifacts came from this tree: https://github.com/gohai/llama2.c-emscripten

TODO:

• CDN for models & tokenizer.bin
• not firm about naming of things ("LanguageModel"?, "manual*"?, "tokens"?)
• would be nice to come up with a Colab notebook to do training/fine-tuning of custom models