Refactor the Rust code

[nikomatsakis]

The Rust code started small but it's gotten rather sprawling. This refactors into a number of crates and creates a cargo alias (cargo rpg) for interacting with them.

A couple of notable goals

• the rust_project_goals library just defines common data structures and utilities, it's meant to be importable by others
• the rust_project_goals_json library defines specifically the versioned API data structures
• the LLM-related stuff is factored out so you don't have to build it unless you run the cargo rpg updates subcommand, as it takes a while

Goals

• allow people to depend on rust_project_goals_json
• shorten CI build times