Add Rust package (binary-only)

[kylewlacy]

This PR adds a new rust package:

• Using the default export as a dependency gives you rustc and cargo, among other standard Rust binaries
• The function rust.cargoBuild() lets you build a crate, including caching dependency downloads and validating the lockfile is up to date

Rather than build from source, the current version of the rust package installs Rust by downloading a manifest file (the same one used by Rustup) and calling the install.sh file for each desired component. This ends up using the official Rust binaries.

To parse the manifest, I had to make two other changes:

• Add typer.record() function for parsing objects based on key/value type
• Add smol_toml package. The Rustup manifests are TOML, so to parse them, I vendored the small-toml NPM package as the smol_toml Brioche package (I made a few manual changes to silence warnings and to match the formatting conventions for .bri files)

As part of caching dependencies properly, I internally pulled in a precompiled binary for cargo-chef, namely to create a "skeleton crate" so dependencies can be cached based only on the Cargo.toml and Cargo.lock files (instead of all of the source code, which would effectively be uncached). I also tried to use this to cache builds of dependencies, but I was thwarted because each build runs in a different $HOME, and Cargo fingerprints the cache based on the source path. Caching dependencies will take some more work