omnibor-v0.6.0

Version 0.6.0 of omnibor brings a lot of improvements! A quick summary:

• We now produce a Rust library, C static library, and C dynamic library, to
  enable linking with the omnibor crate and using it through the Foreign
  Function Interface (FFI).
• The ArtifactId type now has a notion of a "safe name," which is a version
  of the string representation of an ArtifactId which we expect to be safe
  to set as a file name. There are times when you may want to do this, and it's
  good and useful to have a defined way of representing this.
• Moved the previously-available binary crate under the omnibor package out
  to its own omnibor-cli crate. This simplifies a lot of things for us in
  terms of handling releases, and hopefully makes using and interacting with
  the crates simpler too.
• Added support for InputManifest, including building them, and storing them
  on disk! This was a big lift, and means we now cover a much larger chunk of
  the OmniBOR specification.

We have not yet implemented embedding mode for manifests, meaning when they're
created, the target artifact is not updated to reflect the manifest's ID. The
code is built to support adding this in the future, and we've left todos
in the code for anyone interested in contributing!