rust: move Cargo.lock If you do a `cargo build` in rust/, Cargo will create rust/Cargo.lock. Furthermore, the previous rust/hgcli/Cargo.lock appears to not even be consulted because the auto-generated rust/Cargo.lock varied from rust/hgcli/Cargo.lock. This commit moves Cargo.lock to where Cargo expects it to be. While we're here, bump the version of libc to the latest available. Differential Revision: https://phab.mercurial-scm.org/D1844