Generating gn Build Rules

Once you've downloaded the crate, generate the BUILD.gn files like this:

vpython3 tools/crates/run_gnrt.py -- gen

Now run git status. You should find:

• At least one new crate source code in third_party/rust/chromium_crates_io/vendor
• At least one new BUILD.gn in third_party/rust/<crate name>/v<major semver version>
• An appropriate README.chromium

The "major semver version" is a Rust "semver" version number.

Take a close look, especially at the things generated in third_party/rust.

Talk a little about semver --- and specifically the way that in Chromium it's to allow multiple incompatible versions of a crate, which is discouraged but sometimes necessary in the Cargo ecosystem.