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.