Create a single repository for all versionize-* crates

[andreeaflorescu]

Having separate repositories for versionize crates is making the development of the versionize functionality cumbersome because a change in versionize-derive typically needs to be accompanied by a change in versionize. This will happen in separate PRs and requires versionize-derive to be published first.

Testing is also not easy on the versionize-derive crate (crate has 0.9% coverage score). A bug in versionize-derive will most likely be caught only after it is used in versionize.

These problems can be fixed by having both crates hosted in the same repositories using Cargo workspace. The crates can still be published independently.

The same approach is proposed for kvm related crates as well. You can take a look at this issue: rust-vmm/community#98

This is also the same approach used by serde: https://github.com/serde-rs/serde