GGUF file format specification

[philpax]

Closes #220.

Rendered: https://github.com/philpax/ggml/blob/gguf-spec/docs/gguf.md

Defines a complete specification for the proposed GGUF file format, which should generically describe models to be loaded by any compatible executor.

This is a first draft, so there's still some work that needs to be done - I need to fill in the TODOs and clarify a few things. If you have any suggestions for what should go in the TODOs, please let me know!

Changes from the version in the issue include:

• changing of several of the key-value pairs, including splitting them out into per-architecture key-values
• decoupling tensor info from tensor data, and aligning both
• moving the embedded vocabulary into the metadata, so that it is no longer special-cased