As socialized on https://groups.google.com/g/rules_rust/c/kGMg6haEF44, we're going to split rust_library into smaller rules. This is the tracking issue for the effort.

Steps:

• Submit defs.bzl with rules in the desired end state
• Regenerate docs
• Provide buildozer migration script
• Migrate all uses of @rules_rust//rust:rust.bzl in the repository to @rules_rust//rust:defs.bzl
• Update raze and other closely related projects
• Wait for the community to migrate
• Remove the deprecated rust.bzl file. Currently planned for 2021-10-26.