cargo: cache already-parsed Cargo.toml/Cargo.lock
cargo: cache already-parsed Cargo.toml/Cargo.lock

This reduces the number of toml parsings to 226 for completing "std::".
Before it was over 9000! (insert meme here).

The corresponding time has reduced from ~0.6sec to ~0.1sec.

birkenfeld/racer,alkama/racer,phildawes/racer,racer-rust/racer