Refactor README

[Notgnoshi]

Place a larger emphasis on the composability of the tools. Place a larger emphasis on the generated pictures.

Move more detailed discussion to pages specific to each tool? This could become easier with #89?

• [x] Merge each Rust tool's main.rs and cmdline.rs
• [x] Add build.rs that performs CMake build with appropriate rerun-if-changed rules
• [x] Install binaries and scripts into a common install dir
• [ ] Add a toolname.md for each tool next to it
• [x] Add a very brief description of each tool to the main README, and link to the tool document for more details. Include lots of pictures.
• [ ] Add a TOC to each document
• [ ] Add a better introduction
• [ ] Add a note on dependencies (Python, Rust, C++)

TODO: How to cover different interesting compositions of tools? (doesn't really belong in any given tool's README?)