Set up a Cargo workspace: members, shared [workspace.dependencies], resolver, per-crate commands

Documented steps

1. Create a root Cargo.toml with a [workspace] table listing `members = ["crate-a", "crate-b"]` (glob patterns like "crates/*" allowed). A root with no [package] is a 'virtual' workspace.
2. Declare shared dependency versions once under [workspace.dependencies], e.g. `serde = "1.0"`; each member then writes `serde = { workspace = true }` in its own [dependencies]. Members can add `features = [...]` alongside `workspace = true`.
3. Set `resolver = "3"` (or at least "2") in the [workspace] table of a virtual workspace — see gotchas.
4. Target one member with `-p`: `cargo build -p crate-a`, `cargo test -p crate-b`; or everything with `--workspace`.
5. All members share a single target/ directory and a single Cargo.lock at the workspace root.
6. Docs: https://doc.rust-lang.org/cargo/reference/workspaces.html