Cargo.lock is not stored in repository

[koxu1996]

Building this library is not reproducible, and fails due to nightly toolchain incompatibilities.

For years official Rust guides were recommending to not commit lock files for libraries (only for applications), however this changed year ago - see rust-lang/cargo#8728.

Now it is agreed that lock files should be always stored in VCS, but simply not published in libraries artifacts.

[zie1ony]

Sure, we can include Cargo.lock in the next release.